This debate has gone on time and time again.
A Possible general trend to falling audiance numbers
Audiances wanting shows to hit the ground running
When TNG came out, it had virtually no competition in the genre, from the early 90's more and more Sci-Fi was being produced.
Decreasing Ad revenue
Not taking risks and playing it overly safe.
Increase use of time delayed viewing, so those not counting into the final rating.
Let me discuss several of these.
- Greater competition.
For DS9 vs. TNG yes. TNG was on the air with some sci-fi shows (IIRC, Out of This World, War of the Worlds, Superboy) but in the '80s, most of the syndication was sitcoms like Too Close for Comfort, Punky Brewster, Charles in Charge, Mama's Family, et al.
DS9 initially just had Baywatch & Highlander as notable rivals (Highlander's ratings were always meh to weak. If not for international ratings, it would've been cancelled. It was lower than B5 and the other PTEN shows and lower than the formal synd. cancellation threshold). Over on PTEN, Time Trax and later Babylon 5 were competition. By 1995, fantasy genre shows appeared (Hercules, Xena) and there was a glut of syndicated action shows by the 96-97 season (you might want to consider other action shows competition. IIRC, Renegade was up there, beneath Baywatch, both of which were beneath DS9). But, TNG was #1 in syndication during its run and when it went off the air, DS9 inherited that crown. DS9 had that crown stolen from it by Hercules, then Xena, but during its entire run, DS9 was in the top 3 for syndicated shows
(that weren't judge shows, game shows, or talk shows, or cartoons).
- Hitting the ground running?
The pilot got spectacular ratings, Season 1 was very lackluster though. There were many many reports online of people returning by Season 3 or 4, but they didn't reverse the overall downward trend season by season. I think that's a myth or something only applicable to newer shows. With the way entertainment options were in the '90s, say one cool autumn afternoon on the weekend in 1995, is a viewer going to gripe "This 3 year old show didn't hit the ground running. I remember that. I'm not going to watch" or are they going to say "Meh. Nothing else better is on. This is the most interesting thing"? But yes, a dull Season 1 really hurt viewers.
- Increasing costs?
Not sure. Never heard this was an issue with any syndicated series from that time except for Xena. Hercules was the actor wanting to get back to the US and not waste away on the far side of the world (yes, he sure went on to greatness with a US-Canadian sci-fi show and a movie that ended up almost killing him).
- Decreasing ad revenue.
No! Ad revenue grew significantly over the mid-late '90s. This is what partly fueled the syndication boom in action hours in the '90s. The ad revenue imploded over the 2000-01 season (worst year for selling advertising in 50 years. That's what I saw in print in trade magazines online). It was one component of what ended the '80s/90s syndication boom (another was international financing drying up, particularly in Canada a year or two later. DS9, VOY, B5 were American, Hollywood/LA-based productions but hordes of syndicated shows were filmed in Vancouver, Toronto, New Zealand, Australia, South Africa, Florida, Mexico. Even the top first-run show in synd, Andromeda, was almost cancelled despite decent ratings for the time because of financing issues. Others shows were cancelled because the funding couldn't be secured. Notice Andromeda only got a Season 5 with Sci-Fi Channel providing some funding. Stargate SG-1/Atlantis could air in synd because they were second-run).
- Playing it safe.
Kind of funny comment for the Trek show many feel was the only show to take risks until Enterprise with Season 3. The safe route would have been making it much more TNG-esque, not turning Sisko edgier (they went full-Hawk when they allowed him to save his head and grow a goatee).
- Delayed viewing.
People did not Tivo DS9 during its original run, people did not watch it online. If you had a Nielsen box, they might have counted it if you weren't there and recorded it. Not sure how their policies were. Tape-delayed viewing was big for syndicated shows airing overnight (like B5 later on, B5 earlier on in more rural markets, many syndicated shows later on, like in the '00s). From everything I've seen of DS9's schedule (admittedly, I've seen more thorough records for B5 & Highlander than for DS9 past Season 1), the hours weren't that bad for most people.
- Poor timeslots
. Not early on and DS9 was generally immune to it. When DS9 premiered, only Fox gobbled up some independent stations. The real damage was done when UPN & WB gobbled up independent stations to build their network. With the loss of independence, weeknight timeslots used for syndication vanished. But even UPN & WB had weekends still open (well, WB claimed Sunday) and it took a few seasons for these two to claim all weeknight timeslots.
Generally, until the late '90s, in most big markets, the top syndicated shows (DS9, Hercules, Xena, TNG) got ideal spots on the schedule in terms of syndication/their audience. This usually was Saturday or Sunday afternoons, evenings or a weeknight timeslot when they were available. As the number of syndicated shows grew, many got pushed to late-night & overnight timeslots. Only once a wave of sitcoms entered syndication around 1998 did even large markets see the top syndicated shows pushed to odd hours. Even B5 for most of its run, avoided overnight timeslots in the top 50 or so markets with rare exception (Season 4 it started to change and the last 4 episodes were more often exiled to no man's land, a result of B5's ever-screwy schedule).
DS9 got the ratings it did because TNG had exceptional, anomalous
ratings for a sci-fi series in syndication, because it had pretty good timeslots all things considered, and being a franchise show with a dedicated following and having some appeal to non-hardcore fans (the actual number of diehards was well below the ratings. Why would legions of Trek fans bolt from DS9 as Worf comes on board, as the Dominion war heats up, as the final season starts? I use DS9 because overall, past Seasons 1-2, it had far less complaints than VOY, ENT), as well as having top-notch production values for an off-network series, and cable not being competitive in the '90s with broadcast television. Syndication had a lower ratings cap due to off-primetime scheduling, high odds of pre-emption for sports (compared to network tv) and airing on less watched stations (independent, WB, UPN) but during every season, DS9 pulled in great numbers for that field. DS9 would have been renewed for a Season 8 if it wanted one. If Voyager aired in syndication, it would've been in the top 4 (and later top 3) during its run (VOY's ratings beat Buffy's ratings each season they were matched up BTW). Let me put this in italics: DS9's numbers were great for the field of syndication for the time. Extremely few syndicated shows pulled in numbers respectable or acceptable for networks (TNG, Sea Hunt, Highway Patrol).
And even then, their numbers weren't great for networks.
Syndicated market fell apart due to cable competition (especially as it produced first-run content, but mainly due to increasing numbers of people with cable), crash in ad revenue, financing drying up for internationally-funded/filmed shows,, shows being pushed to odd hours (a result of stations reacting to declining ratings by doing something that would push their ratings down even further [because a show's audience will invariably shrink if it's put in an overnight timeslot]), and network shows being packaged after 5 or so seasons for strip syndication or weekend airings (e.g. X-Files, Seinfeld, ER, Friends) which pulled in higher numbers than many first-run shows. This last point is what ended the syndication boom of the '50s and very early '60s. Now, cartoon syndication, that ended in the late '90s for very different reasons
And some shows ended off with among their highest ratings. I Love Lucy, Andy Griffith Show, Seinfeld are the only shows that went out on top, every other show saw a decline for their final season, even TNG. It's just that TNG's final season had very good ratings for syndication.
Simpsons' ratings are way down. Networks had to keep lowering the cancellation threshold due to cable and the internet siphoning away viewers (even internet surfing before web 2.0, Youtube, Hulu appeared was siphoning away tv viewers). All shows have seen their ratings decline. You'd have quite a laugh if you'd compare Lost's ratings to '90s shows and see how much more hype and acclaim it had compared to comparably rated shows from 10 years earlier.