“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.

The downside to the user experience, so to speak, is that the newer OS will be more powerful / featureful, which will require updating a lot of managed policies for places like schools or locked-down businesses. They'll have to evaluate whether they want to deal with that and continue on the new OS and commit to it in the future, continue with it for a very short time then start all over again on windows or something, or just give up the time they were promised.

Of course, I don't think any schools are getting 10 years out of a chromebook, but some businesses might be.