Well you can still install packages transactionally, I use immutable distros often. Say
you had a package that wasn't available as a snap, flatpak, appimage, etc*. You would
start a transaction, install the software, reboot, and if the system boots, transaction
complete, if not... it keeps the old snapshot and things are the same as before. Otherwise
the system is always in a read only state (user data excluded). RPMs wont be gone
completely, but containers is where things are going.
Id imagine getting TDE running on a immutable distro wouldnt be too hard, as long as
everything that gets changed is in user dirs