No one is invincible, not even MS. For a long time the gaming world thought Sony was invincible, but not many feel that way any more. Same with Nintendo before Sony, and Atari before Nintendo.
Even if MS forces Sony out and captures 90% of the market, Nintendo will still be around doing what they've always done and generating profit even with less marketshare.
With success comes arrogance. This was Napoleon's undoing, it was Hitler's undoing, it was Nintendo's undoing, and now it is Sony's undoing... one day it will be Microsoft's undoing as well.
Why worry about it? I'm more worried about Microsoft installinng chips into people's brains and controlling them with their proprietary sofftware than anything, lol. Look at Sid Meier's Alpha Centauri and how the Morganites goal is to get everyone nerve stapled. That's what we need to worry about from MS.