OEMs won't mainline their drivers with all their trade secrets. That's impractical, unrealistic wishful thinking that harms consumers.
The drivers aren't where the important trade secrets are. Those are all in the hardware or in the proprietary firmware that the driver has to upload to the hardware, or in a userspace blob as with AMD's GPU drivers. The code that actually runs in the kernel on the host CPU doesn't reveal any valuable IP given the way most hardware is designed these days.
Nobody expects that. Once drivers are in the mainline, the OEMs don't have to do any work to keep them from getting broken when Linux devs decide to do the kind of refactoring and systemic improvements that are impossible on Windows. All Android OEMs have to do is git pull, make, and send the new OS image off to the same QA processes any other update needs before deployment.