Nothing happens all at once. WotC has to build up their VTT first.
The next thing that happens is WotC pulls support for D&D from all the other VTT platforms. Don't want any competition. Depending on how current contracts are written, that may not fuck with any existing 5E content that's already out there. This doesn't happen all at once, it takes place in stages over several years.
Without the D&D audience, the other VTT platforms go tits-up. It isn't likely that they all die, at least one of them probably survives. There isn't enough non-D&D money to sustain all of them, though.
This will end up with the curious situation where people are paying WotC a subscription fee to play Pathfinder, because all of the art assets are compatible.