“Completely different OS” might not affect the user experience all that much, though, since these devices are for running Chrome and it will still run Chrome. (And be better at other things like running Android apps, presumably.)

We don’t have any details about what the upgrade path will look like and it probably hasn’t been implemented yet, so this is all speculative.

If they didn’t do an OS change, they could still evolve it a lot over 10 years. Chrome could evolve too. You’re trusting the vendor to not screw up your workflow either way.