I use Slack every day in the Desktop Electron app and the iOS app.
Firstly, lack of accessibility is lack of accessibility. Beauty is a distinct concept. Your comment reminds me of the sign on the Berkeley/Oakland border that reads "Animal rights are human rights". Both are highly desirable, but that does not make them synonymous or coextensive.
OK, It's never occurred to me that I wanted a conversation in a different window.
You use ⌘-k to switch channels. Partially drafted messages are retained. I think I prefer that to a mess of windows that I'd have to manage though of course I don't mean to impose my preference on you.
I have the desktop client open and it's using about 800MB memory between the main process and 3 helper processes. I agree it's a lot, but it seems 100% standard for modern applications. Everything seems to be built around the assumption that people have a $2000 laptop like rich westerners might. So I don't agree with it but I wouldn't single out Slack for criticism beyond any other modern consumer tech company.