How easy is it on Slack to contact a person outside of your company? My impression is that it's fairly convoluted compared to email, but we don't use Slack extensively at my org (only for casual internal chats), so I don't know if that's actually the case. I don't see it ever replacing email without a way to conveniently communicate with people in a different workspace.
I think it'd be nice if Slack was setup more like Discord, where users have unique IDs that they can use to contact other people directly.