Here's their vintage/obsolete products info page: https://support.apple.com/en-us/HT201624
This is one thing I think Linux distros could do better: not just advertising upgrades to the next release, but warning when the current release is (soon to be) no longer maintained.
Despite that, a custom ROM feels like having a modern device again. All the marketing-driven little walls are gone.
I have a few WP devices that had more updates than all my Android systems together.
Until Google gets really starts forcing the OEMs to provide updates, nothing will change for the regular users.
Treble is not the solution, as the OEMs are the ones that should provide the updates and certification is only required if devices are actually shipped with 8.0.
Of course technically inclined users will just root their devices, assuming they are willing to trust random downloadable firmware blobs.
Hmmm. Current new iPhone 6s - $449, SE - $349 and plenty of used and refurbished options from $300.
Also some people like to have the opportunity for once on their life to buy brand new things.