Firstly, "video gaming" is really competing against things like reading a book, walking your dog, board games, etc., so it's not like Microsoft can just start jacking up prices and people will have nowhere to go with their time.
Secondly, creating and releasing new games has never been easier. So many small indie game companies are creating great games to compete with blockbusters like CoD and LoL, the ecosystem for game development is plenty healthy, with or without Activision belonging to Microsoft.
Thirdly, they haven't done what you're saying with the games they have released; you can play Minecraft on the Switch [0]. Maybe wait for Microsoft to actually do the thing you're worried about before criticizing them for it! They have had opportunities to be exclusive and they haven't taken them, so it's not so simple as to just assume they will no matter what.
I'm not worried about the industry, but I am cautiously optimistic about what Microsoft will be able to do with some IP that I've loved for most of my life.
[0] - https://www.nintendo.com/games/detail/minecraft-switch/