The truth is that buying a dev to rob games off other platforms doesn't "legitimize" anything. It's even more shitty than timed exclusives. But console fanboys like to peddle that tale to make themselves feel better.
Literally any dev can develop games for multiple platforms, there is no such thing as a 1st party dev. There are only devs that have been forbidden to port their games to other platforms.
The way you see things is probably the problem. You see it as a company purchasing a developer to rob you of that developer. The business is not looking at it that way. They see an opportunity since the dev was up for sale and they evaluated whether having their games under their ecosystem helps to broaden the appeal of their systems. You are not robbed of anything because you as a consumer still have a choice to go to the new system or not.
You see things as if every developer wants to develop for all systems while the developer who just sold to a one platform company feels relief that they only have to code for one platform to make ends meet.
Games are a business and there are real consequences to not selling. Everyone wants that big dollar payout including devs and yes they will make decisions to make sure they get paid just like everyone else. Their is this gamer pie in the sky type of reality where all devs only think about what the gamers want and then their is reality where devs care about what they want and in the process make a product that they hope appeals to a large enough segment of people. Not every Dev or studio wants to be independent. Some like the umbrella of a big corp like Sony, Nintendo or MS footing the bills. If a dev studio or publisher is up for sell then it would be silly to leave them on the table hoping that none of your competitors pick them up.
Purchasing a Dev no matter if they are first, second or third party for a business is a business decision on the prospect of that developer helping them to grow their business and not to please gamers on competing ecosystems. MS has the cash and they see the likes of Google, Amazon, Apple coming in with deep pockets. Hell, even Netflix is coming into the game. The gaming landscape is going to change big time, so gamers better saddle up and get ready for the gold rush.