Looking back at TNG now, it does LOOK very dated... and for the first few seasons it does FEEL very dated in it's sensibilities and dramatic pacing... but I feel like the acting and the quality of the kind of stories they were telling still holds up incredibly well overall
While TOS was narratively grounded as a kind of sixties morality play wrapped up in a campy, action/adventure package, TNG was able to be more subtle with it's social commentary when they tried -- and get away with it mostly due to Patrick Stewart's acting talent.
Where TNG lost it's way was it's devolution into using technobabble to solve a story and increasing reliance on heavy science/cosmic anomaly of the week tales.
They also began to plague themselves with endless backstory episodes, and became more concerned with "filling in" the Trek universe as opposed to pushing the concept of the show further than it had been. This was the start of Trek becoming too insular and bogged down in continuity, and that pattern ended up hurting VOY & ENT down the line.
Watching it now, as a drama, TNG seems so drawn out and talky... boring frankly, compared to today's television. In some ways TOS holds up better because the action tended to drive those plots as opposed to conversation and debate, as was so soften the case on TNG.
And when it comes right down to it, no matter how silly it got, TOS was just more fun... and that never gets old.