The main issue these days is driver support. The PC platform was an anomaly in backwards compatibility, at least historically. I'm not arguing that it's going to be easy for FOSS. It's going to be an uphill battle, regardless of how locked down they are (and I'm just arguing that they won't be that locked down—see the recent M1 Macs for an example; Apple could easily have locked down those systems in exactly the same manner as iOS/iPadOS devices, but chose not to).