Computer are still changing rapidly, and it is impossible to predict what will make any computer 'not useable any more'. In almost every case, the computer continues to be able to do exactly what it could do on its first day on the job, and often a whole lot more with upgrades.
The thing that makes Users set their old computer aside (besides not being willing to repair them) is increasing expectations of what should be easily doable. That is NOT planned obsolescence!
One thing has been the inability of older computers to play Video in real-time. Another has been the inability to support advanced Network and Encryption standards quickly using built-in processor instructions. Another has been inability to do advanced graphics. Another has been processor susceptibility to highly advanced malware exploits.
Apple has previously provided a new MacOS in the fall of the year and continued support for two previous major versions as well. (but they are under no obligation to continue that practice.)