So $50/yr for 4 years gives you ~$150 with $50 extra for shipping or whatever, which gets you a decent Lenovo M700 Tiny with much better performance in both power and power consumption.

I guess. It's hardly an open-and-shut case of "throw your old computer away!" though, especially when this is a worst-case scenario of running a desktop computer at full blast 24/7 without it ever going into sleep mode or being turned off, and when you don't know what the user's needs are. Maybe a mini-PC with basically no expansion just won't really work for them?