[Qwertious]

>But the very idea of software having a pre-defined lifespan is not sensible. Do we put expiry dates on mathematical theorems?

IMO it made a lot of sense when the hardware was changing rapidly - when the speed of new hardware doubled every year or two, maintaining existing hardware for more than a decade or two was often just a bad idea. And new hardware usually means porting or rewriting the software.

Nowadays though, hardware has largely peaked - CPUs are getting faster, but not that much faster. We won't see five doublings of CPU speed (64x increase) in our lifetimes, let alone in a decade. Security issues and repairability aside, today's hardware and operating systems will be perfectly serviceable in, say, 30 years.