Meanwhile I have a 5 year old 5S that I use while traveling which runs iOS 12.1 swimmingly, and keeping older devices running longer was a major topic discussed by Lisa Jackson at the last Apple keynote. I fully expect that this old phone will run iOS 13 as well, and I expect that current iOS devices will have increasingly longer usable lifespans than 5-6 years. There isn’t anything close in terms of manufacturer support on the Android side of things.
In terms of keeping phone hardware useful through an alternative OS, you may also want to keep an eye on PostmarketOS [2].
Its frankly shocking that the Allwinner A10/A20/A64 (which can run Win10!) /H3/H5/H6 has nearly complete mainline kernel support, whereas nearly any of the smartphones made since 2008 can't boot mainline Linux and drive their cameras, screen, GPS, etc. Its a huge failing of the industry.
With regard to supporting tablets, most of the older Allwinner based tablets have feature parity between the mainline kernel and the vendor BSP kernel: https://linux-sunxi.org/Linux_mainlining_effort
Edit: actually, when I say 'latest' I mean 'latest available for the moto g'. I'm not 100% sure that's the very latest version that exists.
It's not. You're stuck with an Android that's 2 versions behind current and are still using the original kernel version that shipped with the device.
Take a copy of a standard 2008 version of Ubuntu desktop and try and install it on a 1998 laptop. I doubt it’d run at all, not without ditching the default desktop.
That's something for the users to decide. Imposing artificial restrictions to software, including forced obsolescence, is never a good thing.
Released 2008. If I get an x86 laptop from 2008, I can install a modern Ubuntu on it with approximately 0 effort. If anything, it'll usually be better than cutting-edge hardware because the drivers have already been developed and released in a stable Linux version. iOS still loses this comparison.