Yeah i agree with you, but there is a limit to community support. The Steam Deck specifically has a big community, but most hobbyists don't like to spend a ton of time maintaining ancient hardware drivers.

I believe my 11 year old Thinkpad T540p still runs mainline kernels too. The GPU is not supported by the 2018 Intel Iris userspace driver though, so I would need to run a legacy driver that does not support vulkan. Its still packaged by Arch, but it does limit my options.

I'd say 10 years until new games stop running with all features, and 20-30 years until it stops running mainline kernels and loses network access to Steam.

Other handhelds with closed-source drivers probably stop running mainline in 5-10 years.

Yes, but this has nothing to with my initial statement.

Plenty of software still supports Windows 7. So literally not everything they made still works, there is no guarantee.