The matter over lifespan appears to be subjective. I am aware of certain Linux PCs that will just Keep On Going, and of course, RISCOS machines NEVER break down - due to the system and hardware architecture I guess, and the fact that no-one writed viruses for them! Insofar as the Mac. v. Windows debate goes, I know a company that operates on Windows as the Government software is Windows only; however, they are forever fighting off viruses, updating hardware and being held to Ransomware. Obtusely, many employees use Mac laptops at home and haven't updated their hardware in half a decade with no slowing down.
If you had to squeeze me, what would I say? Look after your computer and it will look after you. Do not be afraid to try new things - words such as "Ubuntu" and "Linux" sound scary but they're just another type of computer, getting to be quite mainstream now and ridiculously easy and safe to use.