You can install system wide apps by turning off immutabe mode and configurating something. I might be talking out my ass though cause I have done this but I don't remember if it persisted across updates.
You can for sure! It’s just I wouldn’t go around recommending that to just any person because they then might get a little crazy with the freedom, brick their Steam deck install, and then try to say that the Deck is a POS.
Bad word of mouth is always a lot more damaging than spreading the good word that it’s a solid system.
Good point. Though the deck actually keeps a backup. Updates are done to a second partition and if it fails to boot for some reason, it automatically rolls back to booting from the previous good installation. That's why it's really hard to completely brick the system.
But also why with every update all the modifications you did are reverted. Not that big of a deal once you know about it though, I just have a script that installs and configures everything after each update.