As proven by among others Zoom, Whatsapp and now Discord if you build good software people will switch to it. I think you're undervaluing how good the software is. They are 5 years old, started from 0 user base and now they are a huge player and getting bigger by the day. You would have a point if there was something remotely close in quality on the market but not as popular but there just isn't. Discord is the first communication platform which doesn't suck in some obvious way. If Microsoft manage to buy it for 10B and not mess it up it will be one of their best acquisitions.