All of those were a pain in the ass for one reason or another, Discord caught on like wildfire because it was a huge step up from what existed. Pretty much a combination of the best parts of IRC + Teamspeak + Skype.

You don't think Discord has any lockin? For a lot of games, coordinating voice chat these days is "paste a discord server invite in chat". Pretty much any guild/clan/subreddit/group/whatever has a Discord (apart from ones with really old TS servers etc), so if you're in one of those, you probably have Discord. Much easier than negotiating Skype/Mumble/TS3.

I'd say Valve/Steam is feeling a lot more pressure these days than Discord.

I'm not sure Discord will ever be a hugely valuable company, but judging by the fact that every launcher in existence (Steam, Battle.net) is trying to clone their features, I'm sure they can at least sell it for quite a bit.

I was really expecting them to push out of the gaming niche, but with the store it looks like they're doubling down on that angle.