What's actually expiring in these machines? They are built on known technologies like BIOS, ARM, Wi-Fi. In other computers and laptops, I can update the software for 8-10+ years before stuff gets screwy. I get that they can stop updates due to minimum RAM or display resolution requirements, but this isn't that.

This isn't the case for Android because of the SOC blob and how entwined it is with the Linux kernel, but that reasoning doesn't apply here.