hardware running 100% won't last five years
if hardware is not needed to be running 100% at full steam for five years, you can turn down instances on the cloud and you don't pay anything
in 2 years you'll be stuck with the same hardware, while on the cloud you follow cpu evolution as it arrives to the provider
all in all the comparison is too high level to be useful
Five year is a pretty typical amortisation schedule for HPC hardware. During my sysadmin days, of CPU, memory, cooling, power, storage, and networking, the only things that broke were hard disks and a few cooling fans. Disks were replaced by just grabbing a space and slotting it in, and fans were replaced by, well, swapping them out.
Modern CPUs and memory last a very long time. I think I remember seeing Ivy Bridge CPUs running in Hetzner servers in a video they put out, and they're still fine.
if you have spares, spares need to be in the cost, and value lost to downtime stay minimal. but you have to include spares in the expenses. if you don't have spares, 1-2 day downtime is going to be a decent hit to value.
You have a handful of nodes that the cluster can’t function without (scheduler, fileservers, etc), but you buy spares and 24x7 contracts for those nodes.
Did I misunderstand your comment?