i’m an artist that uses twitter, and my feedback is: do. not. do. this. PLEASE.
it is so, so hard to promote your art on sites like instagram and facebook that insist on compressing everything into a gross jpeg. often, the inevitable artifacting washes out details as well as signatures (which contributes to art becoming orphaned, as people with no discretion save and repost it across websites repeatedly, further destroying the quality).
compression is less noticeable on photographs, but for something like a digital painting, it can make a HUGE difference. if you do this, you will remove artists’ control over the images we’re trying to post and add a really frustrating wheel-of-fortune element where we won’t actually know if something is going to look good until we post it.
i’ve read the thread, i’ve read your explanation, i understand and appreciate what you’re trying to do. please reconsider the method. this is not a good solution; find a different way. this is really going to harm the art community and we are already being kicked out or mistreated in basically every other widely-used online space right now. 
eta: i see you’re promising a lot of “better in the future” and “continuing to improve” and sorry, “maybe technology will progress to the point that this isn’t a problem” is not a good enough answer when you’re breaking a feature that we’ve already had for a long time, and alienating existing users in the process. a nebulous “someday it will probably be better” does not help us in the present. reducing functionality that already exists is ALWAYS, always, always a mistake. i think you can do better than this “compromise” (not to mention, it’s not a compromise when one party is being dragged into it with almost no opportunity for input; almost no one in the art community on twitter knows this feedback thread exists because twitter is never transparent about upcoming bad idea implementation until it’s too late to turn things around).
eta #2: i can’t imagine brand accounts will be happy that any non-photographic images they post will get crunched. this also negatively affects things like logos and graphic design. jpeg is just garbage for basically anything that isn’t a photo. :\