Oh, you mean the threshold in mockup size, not the temporal turning point in technology, right? Because I was going to object.
Bingo. Sometimes I assume that my chain of reasoning is obvious and forget to spell it out. Visual FX technology was up to the task of delivering better composites for TOS, but the budget probably was not up to it. So a shoebox dressed up as a spaceship was the proper compromise.
If the Federation shuttlecraft were the size of an Alpha Eagle, then a partial mock-up, such as a door and boarding stairs coupled with some kind of composite effect, might have been used. SPACE: 1999 did that many times, and STAR TREK featured a handful of matte paintings. Still, that did not prevent the Eagle from having mismatched interior and exterior.
I've never done a close comparison of the interior and exterior of Serenity
in FIREFLY, but I would guess that the fit is better. CAD can be very helpful with features such as collision detection. But even CGI can lie. Draco's wings in DRAGONHEART scaled as they opened and closed because actual folding would have fit him like a tent. And the nested rings of The Machine in CONTACT were not "rigged" in a real world fashion. (The movement looked odd at higher speeds.)