Linux

it's not that important if you know what you're doing and don't break your distro.

That may be true for intermediate level users.

Let's go about it this way: Arch for sure is the mutable distro that requires the least fiddling when using it for many years. Much less than any distro that doesn't roll and/or relies on 3rd party repos could ever achieve. Arch only ever has very small hiccups, almost never actually breaks. And yet, after the hundredst time of upgrading the keyring first, recompiling some AUR package because some library changed under its butt or whatever tiniest annoyance, you grow tired of it.

After a decade of usage you know all these things, you have explored every nook and cranny of your OS, the excitement for messing about is over. You just want your computer to take care of itself, because there's nothing entertaining/surprising/interesting in it anymore.

An then an immutable distro becomes very attractive. You get an OS that does its thing, no manual intervention required at all. You can concentrate on the stuff you want/need to do. The OS is not the joyful toy with productivity benefits anymore, just a plain tool. Also here and there you may finally discover some interesting new kinds of bugs or challenges that arise from the new paradigma of containerizing literally everything.