> now you've still got a window where something can still break everything while the filesystem is mounted read/write
I agree, but I'm also sure you would agree that a small window were things can go horribly wrong is much more preferable than than a perpetually open one (sudo and friends stand testament to this).
Please bear in mind that my comments are in no way an attack on your person or one directed at systemd. What I want is to find a workable solution or, at the very least, a compromise to prevent this type of issues from occurring. I strongly believe that protecting hardware is a core responsibility of any operating system.
Would it be reasonable for me to ask you to coordinate with all the major tool developers to gradually phasing out this behavior? As in both the filesystem and tooling continue to support both current semantics and a newer version until the older semantics can be safely disabled. This is how support tends to get phased out from the kernel for obsolete hardware.
Issues of this type have reared their heads many times already and I expect they will continue to occur if the underlying reason is not addressed. As the person who has the ability to do something about it, I urge you to reconsider.