They (both Microsoft and ActiBlizz) pulled games from Steam before, and they're both back on Steam well ahead of this deal. I don't see why that would change.
We've now seen through court documents and transcripts what many of us suspected in that many of these games and studios that Microsoft purchased for exclusivity were Sony targets for exclusivity as well, so if we had to pick one, the company trailing in the market sounds like the better one to get them as exclusives.
I can only see this as better for competition than Sony running away with the high-end console market, because then there's realistically only one console to buy.
All that said about the above, fuck exclusivity in general.