I expect Apple to be there if Microsoft (or Sony or Nintendo) decides to give it up, they are always looking for new markets in recent times and videogames are clearly a big one.
The worst thing that could happen though, would be a 4th player I think.
There will probably always be a market leader with more than 50% market share. That´s how this industry works, one platform sells best and the companies go with it and the platform sells even better while the other platforms sell worse - the gap only becomes bigger in general.
So with one market leading company where is the market for three other consoles? It would need another growth worldwide to make this industry healthy for four console makers.
It happened during the 6th gen, Sega went bankrupt. That could happen to any of the 4 companies then.
It is very likely if the Dreamcast woudl have survived the sales of the other consoles would have been lower. Gamecube at 20 million? (instead of 21.5 million), Xbox at 22 million instead of 24?
The question is how long can companies keep profits when there are more parties, in a business that gets more expensive every generation?
Would a 7th generation Gamecube with the exact same sales be able to keep a company in profitable regions?