I'm pretty scared what userland piece of software will be re-written while ditching backwards compatibility and making the current body of support knowledge worthless. After all, we've replaced the display server (sort of), audio, init and service management, network commands (netplan) if not much more.

My bet would be on a rewrite of CUPS in Rust. Oh, your printer that worked for 20 years is now a useless brick? What a shame, at least now the printing subsystem is secure and blazing fast.