LOL LOL LOL.
You know you're in trouble whenever someone uses the word "logic" in a TrekBBS post.
The basis of my argument has been that the higher warp speeds seen in TOS, seem to be unrealistic(in-universe) for a starship to reach. The reason for that is because just like any piece of technology, there is only so much mechanical stress that the warp engine and other propulsion-related pieces can endure, before they stop functioning or blow up. The warp scale used in TNG, Voyager, and DS9 sets the limit at warp 10 because it's more realistic, and people tend to feel more a part of the show, if they can relate to it.
To help you out.
Okay, Out-Universe explanation:
In TOS, we saw warp speeds by aliens, hostile craft, and sometimes the Enterprise herself with some modifications, hit warp 10, warp 14, warp 9. "Impossible, she can't go that fast!" Scotty would yell, and people would look at amazement and Spock would say "warp T E N" very seriously. But basically it was established that the Enterprise was a high tech ship, and she only went to about warp 8. Except when something crazy happens. But the fans were most familiar with warp 8 being fast, and warp 9 being really, really fast.
In forming TNG, Gene and The Powers That Be, wanted to get away from "the transporter not working", "warp 15!" plot devices, and from what I can see, finding more copies of 1930's Earth. They only ended up sticking with 1 of these premises. BUT the idea was that the Enterprise D was much, much faster than the TOS ship, but didn't want to get into the clunkyness of "warp 18". So the production team re-draw the "scale" of warp speeds, and it was made clear that nothing goes faster than warp 10. Boundries. So please no stories about going faster than warp 10. (That didn't take long). But it was clear that the Enterprise D's warp 6 was a helluva lot faster than the Enterprise or Enterprise-A's warp 6. Speed is a function of design and power. But all a warp "factor" is is a description of speed. Use KPH or MPH, or feet/second. It's a measuring stick for speed and The Powers That Be wanted something a little bit more elagant, but wanted to be clear that the Enterprise-D was really, really fast.
Okay, In-Universe explanation:
The warp scale was changed, or redefined, based on what was learned and experienced from ships going much faster. 10 was the barrier on the scale became an infinite velocity. All other speeds were scrunched into the 1-9.99999999999999999999999999999, so "warp 6" described something much faster than it used to. So did "warp 8". The only technology that changed was propulsion, design, and power, and the understanding of the efficency peaks.
Okay, In-Universe fandom theories (Ships Of Starfleet, Starfleet Prototype, Mr. Scott's Guide...):
Probert's 1701 refit was often in fandom tech manuals given speeds like Warp 8 cruising, Warp 10 flanking, and Warp 12 emergency. A measured speed increase from the design improvements. Other designs in similar publications, and things like the new-build Enterprise-A, were said to reach speeds of warp 14, 15 or 17 for brief moments. The ships were getting faster and faster. Somehow NX-2000 and Transwarp were supposed to change the game, but somehow, the results really weren't as expected. The powerplant and ship design was successful, but it never did deliver on the speeds promised. BUT in failure, it helped re-define how the warp scale was looked at. So around 2300 warp 10 became infinite speed, and all the other warp speeds (including crazy warp 20!) were put back into a 9 integer scale that was defined more by theoretical efficiency peaks than by velocity alone. At this point, hardly any ship could reach the new warp 9.0 - and they wouldn't for several decades. Come to the close of the 24th century, and ships were pressing into the 9.999999 range. This was not helpful from a command standpoint. The scale was redefined again, inserting a new warp factors were only decimals were before.
It's more of a back and forth between what's practical or not. Perhaps having warp 44 isn't as helpful as making warp 44 the new warp 8, but perhaps having warp 9.9999977777777777544444 isn't as helpful as having a warp 12.
Now, to PD:
1.) No one is arguing that warp drive, warp power, and ship design hadn't improved by TNG.
2.) A change of scale, though, doesn't nessesarily describe an improvement of speed, technology, or science.