Generally, if you're buying a server from one of the big vendors the service-contract for it will be pretty cheap until it turns 5 years old. At that point, if you can even get one, it'll be VERY expensive. For our hardware vendor the 1 year service-contract cost at Year 6 is about 50% the cost of a new server. This is how they encourage regular hardware replacement. You are factoring in the service-contract in your ROI calculations, right?
Once a server turns 5 around here it tends to get relegated to test/dev/low-impact roles and we keep a grave-yard of similar machines for parts if it comes to it. Those of us who keep this hardware running frown very discouragingly when entities ask to use old stuff that we have 'just laying around'.
My oldest monster right now is an ancient HP LH3, a 12 year old server, that is performing a single task: monitoring our datacenter UPS internals. The next oldest is a 9 year old that's due to be replaced in the next 3 months. Neither of these have service-contracts, and if they die we will sigh deeply but won't be otherwise inconvenienced.