Comment by dwaite
It will not be able to reliably work with the A partition once B boots the first time - the update process even for many minor releases will trigger local and cloud data upgrades, and there are security reasons to not leave the option to leave older/unpatched versions runnable.
I don't think I've had a software update fail since iOS 7 beta 1 - and that was an issue with updating local data on first boot, so any attempt to revert to the prior OS without a wipe would have been pointless.
I believe the point isn't to let you rollback at will, but it'll automatically rollback if it fails a boot check, so before anything is written to the filesystem. However, it's worth noting that Fedora Silverblue/IoT let's you rollback at will without problem.