There is a single "desktop" user on chrome os - 'chronos'. Even when you login with different accounts, everything is still running as chronos (id 1000).
What happens is that there are separate loopback filesystems, 1 per each "google account", all stored under /mnt/stateful_partition. These filesystems are encrypted (ecryptfs). When you login, the relevant filesystem is decrypted and bind mounted over /home/chronos.
All of this is done locally, no network queries involved. I don't think that you lose your local files if your google account is somehow borked. You just lose the online aspects of that.
But as a general rule, your overall point about not being too reliant on googlopoly is one which should be well taken.
A true laptop-appliance with an immutable-ish OS, decent security and fast/easy updates is actually a quite compelling notion. I'd be pretty uncomfortable with it not being local-first for user data though.
I'm not making a 'Google is evil' argument (that would be a different conversation). I just couldn't bear to trust any corporation with that degree of arbitrary power over physical objects in my possession, regardless of whether or not I'd use their webapps. The power imbalance is just too great. Google (or Microsoft or Apple) is, in practise if not in theory, above any law that can be wielded by individuals.
However, I don't think this can be used until the Chromebook has been initially setup using some Google account.
