I would say that it depends on a lot of different factors.
A computer can be obsolete because you require available spare parts to quickly repair it when something breaks and those are no longer available.
A computer can be obsolete because it is physically much larger, much heavier, much louder or less power-efficient than a more modern computer performing the same task for you.
A computer can be obsolete because of some external change, e.g. when Apple moved from x86_64 to ARM or when some new encryption algorithm or codec is not supported in hardware on that system and the software implementations are lacking in performance or power efficiency.
A computer can be obsolete when its hardware is no longer supported by drivers in modern operating systems with security updates.