What if TOS is not simply an era, from 2253 to 2270, but also an entire starship design philosophy that existed side by side with and during the TMP-era, 2271-2300, and actually became the dominant starship design philosophy in and by 2364, the time of TNG? I say this because I can make a really solid case for TNG ships having more design elements in common with TOS than with TMP ships. Consider, in no particular order, the points that TOS and TNG ships have in common that are exclusive of TMP ships:
1) TOS and TNG ships have the same duck-egg-blue-green hull color.
2) Glowing red matter intakes on the engines.
3) Warp engines are round and "circumferential".
4) Pylons have more perpendicular angles from the hull, whereas TMP pylons are oblique. The majority of TNG ships like the Galaxy, Nebula, Ambassaor, Kyushu, etc have perpendicular pylon angles, the only exceptions being Sovereign, Nova, Akira.
5) Impulse engines are more imbedded into the hull and have less exposure to the outside, and do not have external deflection crystals on them.
6) Saucer tops are more "billowy", not caved-in like on TMP saucers.
7) Copper deflector DISHES.
8) Window that are on-off (i.e. some are lit, some are unlit and black). TMP ships have all windows lit.
9) The name on the saucer is written right across and uncurved, whereas for TMP ships the lettering is curved.
10) Glowing lower sensor dome (on the Enterprise-C at least).
11) Turbolift shafts that you can see from the outside, beside the bridge.
12) Lack of flush vents on the pylons for some designs (the original Connie did not have them).
13) Those copper / black coils on the warp engines right behind the matter intakes.
Now, lets' consider the points that TMP and TNG have in common, exlcusive of TOS:
1) Aztec hull plating.
2) Blue glowies on the engines an nav deflectors.
3) Bridge design.
4) registry font and pennant design.
5) Non-exposed field generators in the warp engines.
6) Sometimes the torp tubes are TMP-style on some ships.
7) Reaction control thrusters.
That's really all I can think of, if anyone thinks of more pleae bring it up.
My point is this: Considering the 100+ years of Trek that we have seen between TOS and TNG, the TOS-design philosophgy for starships seems to have become dominant, making the TMP-design philosophy seem almost like an abberation. But for that to have happened, you had to have had TOS-design ships still being designed during the TMP-era to continue the lineage. Design elements like the circumferential warp engines witt the red glowing intakes. The imbedded and non-exposed impulse engines. The copper defelctor dishes. The glowing under-saucer sensor. The perpendicular angles on the pylons.
If this is correct, then this opens up a whole new can of worms, because then you can have all those TOS-style SFB ships flying at the same time as all those snazzy TMP ships from Ships of the Starfleet and Jackill's, and being contemporaries of each other instead of being ancestor / progeny, or "old" vs "new". I might point out that FASA already had a universe where TOS-style and TMP-style ships existed side by side, whereas publications like SotSF and Jackills gave the overwhelming impression that TOS-style ships had been done away with altogether during the TMP era, at least a new-build designs.
If my idea holds water, that means you can combine the SFB / FASA / SotSF / Jackill's into one big (very big) happy family!