I find, in many ways, that TOS has aged better than TNG. TNG seems to scream 1980s to me.
That is somewhat true. The era from the late 60s through at least the mid-70s is a sort of stylistic watershed for the latter part of the century. Despite some "mod" elements, much of TOS is in step with cinematic and television styles reaching at least as far back as the 1940s, so it's snugly in the mainstream in many respects. Men's hairstyles, for example (disregarding the sideburns).
In that regard it's probably just as well that Phase II
was never made. If you look at a lot of the set and costume design for that - even the font selection for some of the promotional art - it is so-o-o
"disco era." There's even a bit of it in ST:TMP.