Lots of SoCs are "open" in the sense that complete documentation including programming manuals are available. With couple man-centuries of developer time each, you could port Linux over those SoCs. but that doesn't count as being "open". On the other hand, there are a lot of straight up proprietary hardware that are considered "open", like Raspberry Pi.

Which means, "open" has nothing to do with openness. What you want is standardization and commoditization.

There are practically no x86 hardware that require model-specific custom images to boot. There are practically no non-x86 hardware that don't require model-specific custom images to boot. ARM made perceptible amount of efforts in that segment with Arm SystemReady Compliance Program, which absolutely nobody in any serious businesses cares about, and it only concern ARM machines even if it worked.

IMO, one of problems in efforts going in from software side is over-bloated nature of desktop software stacks and bad experiences widely had with UEFI. They aren't going to upgrade RAM to adopt overbloated software that are bigger than the application itself just because that is the new standard.