If all games became multiplat no one would ever buy more than one machine. Game systems would be like Blue Ray players. You would just buy the cheapest one that has the features you want. Great for the consumer, but not so great for Microsoft and Nintendo and Sony.
If just Nintendo made their games multiplatform that would be the end of them. The only reason to ever buy more than one system is for the games you can't already play, the exclusives.
Let's say Microsoft gets GTA6 to be XB1 exclusive. You don't think that would dramatically swing sales for them? Or even the next Call of Duty? Come on, of course it would.