Not to rub salt onto the proverbial wound, but TNG DID have better special effects than TOS.
Still, I found TNG better than TOS because of its stories AND SFX.
My 'controversial' opinion of Trek is that the 24th century Federation as seen on TNG, DS9 and VOY should have been portrayed as massively FAR more technologically advanced compared to what we actually saw given that technology and science evolve exponentially, and in a collection of societies like the Federation combining numerous technologically advanced species in cooperation, sharing, no money, etc. would propel science and technology forward to quite literally 'ridiculous' levels (but good ridiculous).
Unless making progress becomes progressively harder when technologies max out and no fundamental new principles are discovered, and we are limited to making refinements. Planes changed a lot more between their invention and the 1950's than they did since then (I think, even though there is still progress). The same might happen to e.g. computers if we hit some fundamental limits.
I'll agree though that this doesn't seem to be the case in the Trek universe, as Our Heroes they regularly encounter New Amazing Tech, only for it to be shelved and rarely or never to be seen again.
True, but sometimes they reach plateaus. Two examples: one, the internal combustion engines of today still work on the same principle as those of the mid 20th century. Two, the 9mm Luger that our troops still use today was invented in 1908.
The basic principles, yes, but aren't the engines we have today significantly more fuel efficient and less polluting than the ones back then? Progress can continue even if not visible.