I've seen a lot of gamers mention this, but I'd wonder why it hasn't happened yet especially as there's a set of gamers that seem to love centralizing on steam already. One massive downside I can see is that the various public social areas of steam like the game forums are already a cesspool (and have been since they were vBulletin based) and I don't see that improving with more users in near real-time chat.

Discord getting used as a knowledge base or download source for some areas is already seen as a convenience for those involved or a single point of failure by many 'outside', I wouldn't want to see more of PC gaming moving to one place.