What if I'd argue otherwise - that download managers (including plain HTTP(S) downloads) shouldn't be a part of browser, but a separate pieces of software?

I suspect that in modern "we don't download anything, we watch/read/run this on the web" times in-browser downloading is bound to be second-class citizen that vendors would not pay any significant attention to beyond the simplest scenarios. Basically, the torrent support will be barely functional, just like the current HTTP downloads are. If they're split in a separate program (buzzword: microservice) they still have a chance, though.