>monopoly
Wrong. Sony already effectively had one for the better part of a decade now. They were just worried Xbox was actually gonna try to do something when they bought out Activision.
>What happens after
Inflation and a fight to survive. Sony in leaked documents specifically state they aim for poor gamers by enticing them with high powered hardware. They're greedy, but they're just trying to navigate the market too like us. So, some sacrifices must be made. No more concord.
>Who comes in
Hopefully Valve. But I don't know to be honest. This shit takes a really long time to plan for. Having some sort of announcement a year beforehand though would build up hype.