If you really wanted to, yes. You'd have to be able to deal with the eventual inconsistency involved with overriding a distro default of sorts (Plasma), but it will run just fine and won't be overwritten.

Your particular wants have nothing to do with the distro being immutable, but more that it's a productized distro for a specific platform. Your complaint would also work for phones where you want to ditch Android for a stock Linux install. Sure, it'll most likely run, but you need to deal with what all that entails when not built for the specific thing the device is built for, which is why the entire product platform exists in the first place.

https://dev.to/fstanis/how-to-install-cinnamon-on-your-steam-deck-1boe