It's not like everyone has a gaming PC ready to use whenever MS has an exclusive, this "cannibalism" only happens among core enthusiast gamers, and barely even there, which are a tiny part of the whole market. Even here on this enthusiast board there are already 30% of console gamers who don't have a PC going by polls. Outside of the enthusiast groups I wouldn't be surprised it this percentage went up to 80%.
Microsoft's biggest battle is the Xbox brand versus Playstation brand, and that fight is not easily won without any clear advantage that could change the talk on social media etc. Without something that could get mainstream gamers to wake up and realize Xbox is kinda cool too it will end just like this gen. The Bethesda purchase could become big in the future but once any of their games are out it's too late, the market will have already made their choice. At most it'll be like Xbox One X, everybody knows it's great but few are willing to swap considering they're already so invested elsewhere.
But if the gen starts out by having XSX win every single DF face-off in a clear way... Then things could become interesting.
Small differences in third party titles will not be able to make a huge dent into users with PS4 libraries and friends on that platform though.