First: it's not five years. It's five years if you posit that macs are magic and use no energy[1]. In practice they're 40-70% the consumption of a competing desktop (depends on usage and specific model, yada yada yada). So figure a few decades or thereabouts.
But even so: I'm not sure I know a single new-device Apple customer who has a single unit older than five years. The comment about power implied that you'd make up the big Mac price tag on power savings, and no, you won't, not nearly, before you hawk it on eBay to buy another.
[1] And also that you posit that the device is in a compute farm or something and pumped at full load 24/7. Average consumption for real development hardware (that spends 60% of its life in suspend and 90% of the remainder in idle) is much, much, much closer to zero than it is to 300W!