As far as monopolization is concerned, MS isn't actually very close at this point. MS could purchase another large publisher without much concern. Whether or not Sony buys a publisher doesn't matter as to whether MS is a monopoly. Also, there are far closer things to a monopoly, such as Alphabet with search engines.
However, we are starting to get dangerously close to the beginnings of an oligopoly. This is very common with other products to begin with, and there are relatively few laws that prevent it and even less that are taken seriously. With the precedent set, Amazon, Google, and/or Apple absolutely will be stepping in to buy up publishers. And I'd be willing to bet they'll buy up a console manufacturer too. The majority of all publishing will likely be done buy three companies in the future, all acting under different labels to give us the illusion of choice. It's not a good look IMO