Messages on slack can become threads if you choose to reply in a thread instead of a channel, so you can have a channel with only top level comments and all discussion in a thread. Everything is just as archivable and searchable as any email client I've used, even more so because I can use the channels as additional search criteria.

I get emails for calendar invites and git notifications, both of which have slack integration. Other than for password recovery and account creation have very little use for email