I'd have a much harder time selling Discord because of how they market themselves. They could make "discord for business" that is literally the same app and have great success.
Other than that my only complaint is that it's a bit electron-y.
Discord is sometimes known as "slack for gamers" so you're effectively describing Slack :)
Effectively all I want is a lean chat/voice app that’s also ideally free. This is pretty easy - but when you throw in the other Skype features (group voice, screen sharing, mobile apps) as requirements, it’s actually pretty hard to match in the free tier.
Skype is $0 regardless of company size and the resource use is pretty reasonable.
On that topic: Why aren’t more companies using discord instead of Slack? Looking at feature matrices it seems discord does in the free version a lot of what Slack does in paid (e.g group voice)?