That's not really how this works.

These things have a specified lifetime of e.g. 2 years. That means, each component is built and designed to last 2 years with a probability of 99,9%. That in turn means, that there's a high likelihood for the device to survive much longer then two years. Adding additional safety margins for a designed lifetime of 4 years will make the whole thing much more expensive and maybe even less capable, simply because it's going to be heavier.

And additionally, NASA engineers often enough hack devices to work much longer. The Kepler telescope for example had one too many reaction wheels fail and was thought to be dead, but some clever engineer find a way to use the remaining ones to still do some science.