To me the difference is that I was around 13 when TNG aired, so I remember living in that time and how hyperfuturistic the series looked back then. A look that has long since been become obsolete, so, yes, TNG looks dated to me.
TOS, on the other hand, was made before I was born, and that series already looked 'old' to me the first time I saw it.
The side effect of that is that its look never started to 'age' on me or started to look 'dated' to me because it was already old. So to me TNG has dated badly and TOS just is what it is, and I think I simply don't evaluate it on the aspect of 'datedness'.
I wasn't watching it at the time, but I was 8 when TNG started. I didn't catch TNG until mid-way through. And this is where the five years between our ages makes a difference.
The '80s were a decade that
technically I was around for, but I only remember half of it, and what I
do remember first-hand is from a child's perspective. Most of anything from the '80s that
wasn't a cartoon (or geared towards kids), I didn't discover until after the fact. It didn't help that I had parents who wouldn't let me do or watch a lot of things. By the time I had my own musical tastes or my own sense of style, it was already the '90s. So, by the time I was watching something or listening to something from the '80s that was geared towards teens or adults, it was already "old" to me too. It was always old by the time I was aware of it. I didn't have a feeling of, "I remember that!" I had a feeling of, "So this is what I missed out on!"
So, I always thought the first half of TNG looked old. The second half didn't, but it still carried over from the first half. The whole, "I thought it looked new at the time, but now it looks dated!" feeling other people have with TNG, doesn't
fully hit me until with DS9 and VOY.
It's one of the two main ways I'm out of synch with a lot of people here. The other main way is how when they talk about how SNW is "the best Star Trek in decades!" I've had that same
type of feeling too, so I understand the sentiment... except it was with
Discovery! Five years and two live-action series earlier! And that momentum I felt carried over into
Picard. So, when people are like "SNW is the best ever! Star Trek is back!", in the back of my mind I'm always thinking, "Where the Hell have
you been?" It's always at the tip of my tongue.