Lots of use cases become impossible without it That may be the point. I doubt a majority of users ever go near Disk Utility (certainly not ditto, rsync, etc)

Lots of use cases become impossible without it That may be the point. I doubt a majority of users ever go near Disk Utility (certainly not ditto, rsync, etc) MacOS sand-boxing won't give the user write access to the System folder, anyways. Apple management might not see the point in a user booting externally, when they could move their User data to an external drive, instead. If Apple walls off the ability to boot off an external drive, it's bad for 'power users', but good, in some ways, for Apple: stronger lock-in, tighter security, easier to sell higher-spec'd Macs (ie: with larger hdd's), and more iCloud subscriptions (cloud storage).