As with most of these "lose money now, make money later" startups, I think Discord's longterm goal is to get bought out.

I don't see any way they can turn a profit. If they charge for hosting servers, they'll lose a large number of servers (people will just consolidate). If they charge for quality, users will start leaving for the oldguard services (Teamspeak/Mumble).

Persistent text chat is definitely a plus, but all it would take is for one of the slack clones to have a mumble client built in for there to be a free/easy alternative. Or people to just go back to a groupme/whatsapp and a voice service.