Reasonable would have been to deprecate support in 10 years, not 8.

Saying "yeah, we promised you 10 years of support, but if you just bought yours the last two years of that will only be supported if you switch to a completely different OS. But happily some of our devices have a direct migration path (if that's not yours, deal with it). Better hope the features you need will be available in our new OS by then, because right now they are not" is a major rugpull and not reasonable at all

“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.