[rlpb]

Assuming you're using Crouton or something similar on your Chromebook, your Ubuntu installation is inside a chroot. Chrome OS can be updated and just leave that directory alone. The important part of this is that Ubuntu itself is not being updated.

You could do this on an Ubuntu phone or tablet I suppose, by for example running a secondary "traditional" Ubuntu system inside a container in a read-write directory on the read-only system. I don't know if this will be possible from day one, but it's certainly possible technically if someone implements it (like how someone has implemented Crouton). I doubt you'll see the same level of integration (for example with the tablet UI seamlessly moving over to your desktop) if you do it this way, however.