Buying a huge company is not such an easy task as you make it sound. The Japanese govenrment themselves could step in and impede the deal if they wanted, if it was in the interest of the nation.
I wouldnt mind at all tbh. It would make my console purchasing option very clear and would save me money on buying another console. I would go Nintendo only.
Anyways... when buying a company you have to factor in the profit you are going to make by aquiring it. The Xbox division still hasnt turned a profit overall since it started i believe. Why sink even more money into it? I bet the greedy shareholders want them to go more into mobile and phones and that crap that is easy money (supposidly) than to have microsoft aquire Sony and not make a huge profit from it. Their shares would probably drop in price. I dont think they would like that.
If companies were run by people who only care about the business, then yeah it would make perfect sense. But companies are run by opportunistic people that only care about their own profits.