I think the conventional wisdom (and my opinion) has TOS being excellent from the get-go, but losing traction in Season 3 especially. Lots of factors there, due to the change in showrunner, production costs, writers being replaced, and lack of overall support from the studio and an understanding amongst the cast that the show was on its last legs when it was placed in the dead zone to air.
TNG, DS9, Voyager, and Enterprise all got better after the first two rocky seasons. TNG strengthened their writing, DS9 got the Dominion War in gear and began heavy diving into serialization, Voyager had some showrunner changes and got its best stories as well when they delved into Borg and Hirogen territory (as opposed to Kazon or VIdiian fare). Enterprise completely revamped its showrunners for the better, and learned from both DS9 and Voyager. TNG, DS9, and Voyager all had issues with Season 7, lower quality and higher pay for the actors seems to have a correlation.
Only Discovery has reached Season 3 of the new shows, and had a complete series revamp. I, personally, prefer the first season overall, but it seems I'm in the minority, and everyone is praising the new direction. Lower Decks will be going into their third season by the end of next year, and has already promised a heavier focus on storytelling and story arcs, as opposed to the constant barrage of in-jokes and reference comedy from before. As shown in the final episode of Season 2, it's becoming its own thing without relying on past Treks to prop it up, and that's a good thing.
Nobody wants to make a bad season of any TV show. It's just, by the time of a third season, creators have enough input from their audience to figure out what they want and what they can provide, and it seems that Seasons 3 through 5 are probably the highest quality, whereas the last seasons suffer from creative fatigue (a "Seven-Year Itch" so-to-say).