"Many systems get by just fine with a minimal ELF or multiboot loader and a firmware-provided devicetree specification" Not really, none of those ecosystems ha

"Many systems get by just fine with a minimal ELF or multiboot loader and a firmware-provided devicetree specification" Not really, none of those ecosystems has a fraction of the device variation that x86 has. When they do (arm) its a giant mess of incompatibility and non working hardware. Modern DT's are basically still tied to the linux kernel the same way that the old arm/platform descriptions tied firmware id's to individual kernel configurations. Which is why the answer to so many arm problems is "match your DT to the kernel revision", god help you if your trying to multiboot a *bsd/etc as well. PS: Openfirmware is basically dead, that might have been a valid answer in 1998, but even IBM/etc provide alternative boot mechanisms for linux/PPC at this point. The only thing that comes close to a current replacement is UEFI.