Just to pick up on your second point, you can alter the roundness of a single corner:

https://www.youtube.com/watch?v=IzXXmN9AoR4

Well, it's probably simply because there's a million other tools out there that do a way better job either at rotoscoping or generally toon animation, so Adobe may have one day realized that in order to keep up, they would have to invest so much it would hold up overall development of AE. That and of course these workflows tend to be highly specialized to begin with and even if AE had more advanced paint and masking tools, the open question is whether people would actualyl use them for their work in order to sustain long-term maintenance. admittedly it's a bit ov a chicken vs. egg problem as in light of AE not having these tools it is nearly impossible to gauge how successful a renovation of this toolset could be. Anyway, my impression is that most artists simply have moved on to other programs for their roto work or painting jobs in the cases where Rotobrush, mocha and the limited paint tools that exist don't cut it.

Mylenium

After Effects' drawing tools are definitely much more limited than Illustrator's, but you can always create your art in Illustrator, import it to After Effects, then convert it to a native Shape Layer. There are caveats here as not everything you do in Illustrator is supported by native AE shapes, but this is a very common workflow. Additionally, the extension Overlord is really popular for sending art directly from Illustrator to AE and it can even do things the native AI > AE > Shape Layer workflow can't, like recreate gradients. It does cost $45, but if this is something you do a lot you'll find it's well worth the price: https://www.battleaxe.co/overlord

As for enhanced native tools, I suspect the drawing tools are not high on the AE team's priority list, as performance has been the main focus for the last couple years, but you can always file a feature request at UserVoice here: https://adobe-video.uservoice.com/forums/911311-after-effects. In fact, there are likely multiple existing requests for better drawing tools that you can search for and add your vote to, rather than creating a brand new request with only one vote.

This may help you.

Paint tools: Brush, Clone Stamp, and Eraser

Regards!

Let me move this to the After Effects forum for you, which is the appropriate forum for your question.

The Using the Community forum is for help in using the Adobe Support Community forums, not for help with specific programs. Product questions should be posted in the associated product community.