Re: All very interesting.
AFAICS it looks very much like emulating the Windows experience.
Current Linux practice is along the following lines:
libwhatever is used by a lot of executables - low level stuff such as the command line shell and the graphical shell, the command line and graphical file managers and higher level stuff such as a browser, a couple of graphics applications, an office suite and others*. Some vulnerability in it is fixed or maybe some clamoured for extra functionality is developed. The new version is placed in the repository and a user's regular update process discovers it, downloads it, either automatically or by user approval, and everything that needs it just uses it on the next invocation. It's very quick and the only things I've ever seen need a reboot are a new kernel and, on one occasion, a very low level service. In practice a few small updates might pass through every week, a larger batch of programs that work together every few weeks and a new kernel every few months.
As I understand it the immutable version of doing things is that in order to do this an entire snapshot of the core OS has to be created, installed and booted. I doubt this is going to happen just because libwhatever has been changed because it will be disruptive so it will only happen at intervals (monthly? every 2nd Tuesday?). If libwhatever has security issues the core is going to have to live with it unless an out of schedule update is issued. This becomes very Windows-like AFAICS. Maybe this is the intention - make it familiar with a lengthy, disruptive update happening every month. There's also the added detail that AIUI those higher level applications, the browsers etc, aren't going to use the libwhatever in the immutable core, they're going to have their own version in a snap, flatpak or wherever which is a bit of a lottery because they might get the newer version faster than the core if it's released more quicker** or never if the packager never gets round to it.
* In a few cases applications might have their own version for some reason and the launch process has to arrange to use it. It will be stored somewhere out of the way - an application-specific directory tree under /opt is the accepted place or /usr/local if it was locally compiled from source, Like snaps etc it might get updated on a different schedule to the main system implementation. If you're Ubuntu or whatever this suffers from NIH syndrome.
** What would be the schedule for releasing these? As and when ready as per current practice or alongside the immutable core?