You can actually look at history and see what happens when IBM tries to wrest control of the PC platform back with the PS/2, which was a flop with consumers because it wasn't backwards compatible enough with IBM's own previous PCs or the wider PC market that developed. A bunch of PC clone manufacturers got together and came up with the EISA bus standard so they wouldn't have to pay IBM license fees for MCA, and made it backwards-compatible with ISA cards people already had. It was successful enough that IBM ended up adopting EISA for some of their PCs.

The other notable thing about the situation is that three companies ended up simultaneously responsible for a large part of the PC platform, originally -- IBM, Microsoft and Intel. They all worked in various ways to encourage competition to each other -- the reason we see OS competition on the PC platform is that IBM and Intel both found it in their interests to allow other OSes on the platform to reduce Microsoft's leverage over them. IBM in fact created one of the competing PC OSes out the gate, OS/2, which was originally an IBM/Microsoft joint project until they started feuding. Now, OS/2 is dead, but IBM's interest in being able to support their own OS instead of Microsoft's is a big reason the PC platform was built in an OS agnostic way. People criticize UEFI for locking down the PC platform more than the previous BIOS implementations, but UEFI is still _way_ more open than basically any other platform, most of which don't have a standard for bootloaders at all. It's really the absense of a standard for bootloaders that keeps most Android phones locked down. Two Android phones from the same OEM might have different bootloaders, much less two phones from different manufacturers. We've yet to see an alternate OS with the resources to support implementing their own bootloaders for a majority of Android phones.