Time to Replace Hardware Windows 2003

I have four servers that are running windows 2003 server, in various roles. The servers are HP proliant DL360G3 AND DL380G4 servers that were good quality when purchased (dual/quad core Xeon 3.06Ghz, 533 bus, 3G RAM, RAID), but there is not currently enough disk storage.
My outside IT tech is recommending new hardware because of age, although there are no serious events that have occurred, except one hard drive failure.
What’s the problem with keeping “old” hardware that is not experiencing problems?