Well sure but... well.
The 80s had all that sort of mass produced low effort junk too. Some of it may not have been as mass produced as modern stuff like TTG, but that has to do with changes to the industry in many respects (computer aided animation greatly reducing the cost of work and the internet allowing offshoring of animation production to Japan and S. Korea with quick turnaround time and greater producer involvement). I mean,
look at this list and bear in mind that the majority of it isn't fondly remembered and was just junk shoveled out to sell toys.
The Disney Afternoon shows and the other "classic 80s cartoons" are just that, the classics, the ones that stood the test of time for us to become nostalgic for. Likewise shows like Avatar: TLA and the other ones you mention are likely to withstand the test as well (though I have my doubts on SU, but that's another thread), but again, they are the really good shows among a sea of drek, just like usual.
It's like the question of "why was music in the past so much better than modern songs?" It wasn't, it's just that the music that survives from previous decades usually is the really good stuff that people keep going back to because it was really good, while the junk stuff is left by the wayside and forgotten. This happens with all media, and has been happening since we first started recording media in a form that allowed redistribution.