Just to add to the use cases - the upgrade to Sonoma broke one of my critical apps - like others here, I've invested heavily in a high spec machine for the purpose of virtualisation, and not being able to run an older version of MacOS is bonkers (aside from the time I just wasted wondering what the heck was going on !) so +1 for this feature. I'm left hoping that my vendor fixes the issue soon otherwise I have to downgrade, and that's never fun :-/