Many servers run 24 hours a day for months and are only rebooted for patches or other regular maintenance. I haven't seen a server have issues(from heating and cooling) - even after being shut down for a few hours. Not saying it can't happen, but I haven't seen it. We just did a major switch upgrade in our server room, and shut down every server(about 90) for about 8 hours. None of them had any issues booting back up.
The reason most people replace servers is because the hardware gets obsolete. I've had many servers that have run over 10 years without issues except for maybe a hard drive. I've also seen a couple of cases where a single component goes out, and it was cheaper to replace the entire server than to replace a single component.