Why would I use a system that isn’t supposed to change if I want to change it?

There’s a bunch of benefits, atomic updates, intrinsic rollback, security of immutability, safe automatic updating and it goes on. Some things are not quite ready yet, e.g. things like sddm which should probably install themes to /etc (which they’re working on), so as often happens in linux, workarounds ensue. Making one directory mutable does not destroy all the benefits.