For the past couple of decades at least, supercomputers are decommissioned when newer HW becomes so much more power efficient that it's cheaper to buy new hardw

For the past couple of decades at least, supercomputers are decommissioned when newer HW becomes so much more power efficient that it's cheaper to buy new hardware than keeping the old one running. Usually that lifespan is in the range of five years or so. Of course, if you have free power (or say, the power bill is paid by a different department), sure go wild and keep that old darling running.