I suspect space probes have a pretty steep bathtub curve. Lots of potential failure points early on, then large stretch of low risk of failure, followed by incr

I suspect space probes have a pretty steep bathtub curve. Lots of potential failure points early on, then large stretch of low risk of failure, followed by increasing failure risks as it gets old. So when NASA set a target minimum lifetime goal, it puts them quite far into the bathtub curve, long after all that initial risk goes away. If it makes it to it's target lifetime, chances are it will last a lot longer.