The games would just leave Steam. The big publishers want their own platforms and launchers anyway.

The big publishers already have their own launcher and platforms and are increasingly moving back onto Steam because they see higher PC player counts and sales when their games are there

That's not the trend that we're observing. As much as publishers and developers want to control their sales channels, the current trend is for them to move towards Steam, not away from it.

The more likely outcome is that developers would segment matchmaking into people with kernel-level anti-cheat, and people without it. This seems fair to me.