We also need to remember that we have been told multiple times that Xbox is way more "money-safe" than most other publishers/companies. There isn't an unlimited budged and if there are budgets they are taken very seriously at Microsoft. Multiple Xbox studios have talked about that.
When we start talking about buying publishers we have to think of not only the initial cost but all the salaries that have to be payed off. Also layoffs, which always happen in this situations (publishers have a lot of redundant staff when they get bought). I really don't see Microsoft buying a big publisher in the near future, I would expect they would want proof that their recent investments will payoff and then increase in size again.
I do hope they buy more studios in Europe like Asobo though.