As long as it's run within its thermal limits (i.e. not being severely overclocked and has adequate cooling/ventilation) 3 years or even 5 years isn't an unreal

As long as it's run within its thermal limits (i.e. not being severely overclocked and has adequate cooling/ventilation) 3 years or even 5 years isn't an unrealistic lifespan. The analysis makes this, and a lot of other, implicit assumptions which in general seem reasonable.