Well unlike certain anime elitists on the internet, most people don't think animation looks dated so easily.
Hens why shows like adventure time and flapjack were still fairly popular, even though their artwork is crude in comparison to avatar.
And the realistic motions, the fact that all of the bending in that show was based off of a martial art of some kind helps. Example: Airbending being based on bagua.
That's the thing I'm curious about. I can't see how it can look dated whatsoever. Neither do I see shows like Courage the Cowardly Dog, Invader Zim etc as becoming dated because I was exposed to them early on. All of these also occurred after 2000 (I started really paying attention because lets be real, I can't remember anything I watched before 5). But if you were to look at Pokemon, you can see clear differences. Compare the first season to when it was the Advance season (the last one before I fell off the Poketrain) and it is clear as day. However at some point they just all looked consistent to me or the differences weren't enough to affect my enjoyment. I have to admit, as a kid, I skipped out on shows like Johnny Bravo, Dexter's Lab etc due to the way they looked; perhaps it may have had to do with the plot but I can distinctly remember that the quality was somewhat off putting.
I'll give a more obvious example. Toystory. Since I assume we've all seen 1 we can watch 2 and 3 etc no problem. But a kid who is 8 now and has only seen 3, can he still enjoy 1 to that same level?
tl;dr
People's enjoyment of animation is largely dependent on what they were exposed to first during the time where they actually had thoughts (>5). We can more easily accept newer animation then old; it is an arrow that moves in one direction. Newer generations will have higher standards then the past and may find it difficult to get into what we have watched in the past.