I think one should also take into factor that not only software(at OS level), but the hardware(including GSM base-band) is being continuously deprecated in favor of newer and faster(?) hardware. Eg. 5G is pushed aggressively at the cost of lower speeds of 4G and broken connectivity of 3G.
Despite this, if someone is ready to work with a deliberately throttled network, then there is full ecosystem of apps and services which continuously target newer OS versions, in order to get access to better, faster, more secure libraries and OS APIs. If the case is not about smartphones, then the dumb phones don't need any OS upgrade to continue working. A caution here is - those phones would not be secure to vulnerabilities found afterwards.
Similar situation prevails in Computers as well(stopping OS upgrades and associated browsers/apps stops working after some years on that OS) but the lifespan of a computer is longer than that of a phone.
Regarding custom OS -- getting the latest version of Android work on older phone hardware is already tedious task. It requires rewriting the malfunctioning drivers to be compatible with the new OS APIs. The hardware vendors add to the misery and don't release workable documentations on how to approach writing drivers for a different OS. Now, multiply this effort for all sorts of different hardware made by different vendors! I believe a general purpose phone OS which would support such a large variety of phone hardware would be a humongous task.
E: After reading the petition -- I think for the sake of not wasting a decently working hardware we can have some sort of jailbreak. But without a proper working replacement OS after the jailbreak, the situation would not change much. And, if one can provide such reliable OS, it is a potential company idea right there!