I don't actually ever remember hitting a DM length limit on reddit. I do know that reddit had thread/comment length limits which I hit all the damn time haha. But I don't recall ever hitting it in dms.
As for the > convention... sure. I don't doubt that maybe some older email clients worked like that. but modern day you can't really write in markdown style. and I can't seem to figure out how to get gmail doing block quotes lol.
but yeah I'm not really an email person. I prefer reddit-style markdown forums for asynchronous discussions, and live instant message like discord for live chats. email does neither of those, but it's just used because it's ubiquitous unfortunately. email is.... actually really bad. every thing requires it to signup, and then just spams your inbox. no one actually uses it to discuss/chat (and it's honestly terrible for that). so it ends up just being the way you verify your account after signup which is a dumb system. for password recovery I guess it works. it's also nice to get purchase confirmations in email. but other than that it's ehh.
tbh web as a whole really needs a rethink.
ah, then yeah you're right. it's possible in theory, but in practice not really. and it's not even desirable for most.