Yeah, theoretically they're the only company I can see entering, but like you mentioned why would they? I think they're fine with mobile and creating a marketplace there. With smartphones getting stronger every year they could create a store that can provide a AAA mobile experience.Apple is not going to step into the console gaming market. They make too much money on mobile gaming to bother with it. Hell they’re trying to put high production games on iPhone now.
If Microsoft exits Sony will gobble those customers up and even if another company wanted to enter the space they’d be at a weaker position than Microsoft is at right now and it would be much more difficult to change that.
You gotta remember when Sony entered the home console market the entire industry for the most part got behind them and pushed them. For a new player that won’t be the case. No new player is entering console gaming and even coming close to completing with Sony.
The entry into this market being difficult is why people shouldn't want Microsoft to exit because even at a weaker position they can still check Sony on certain things