@nekohayo I don't have experience with other file browsers, but surely there are others that support s/ftp? You can add IMAP/SMTP trough Online Account, even though that's only used by e-mail clients, and most people probably just use one e-mail client at a time.
I think the most intuitive thing would be to have all online accounts settings gathered in one place, not something you have to look for in different places based on which app uses it. If we're to have Online Account in Settings at all