The down-side of the robotic explorers that keep working far longer than anticipated is that they also make demands on the budget for longer than anticipated. While Mars Opportunity and the Lunar Reconnaissance Orbiter still function, they aren't in the 2014 budget.
LRO and Opportunity are happy to keep taking new observations, but are we willing to keep funding them?
Casey Dreier wrote an article for the Planetary Society on the unexpected consequences of successful missions on NASA's future exploration programs, outlining how the robotic explorers that keep on going aren't entirely a good thing. In it, he points out that if these continuing missions keep getting a slice of the NASA budget, then that limited budget isn't going into developing new explorers. A consequence of the unmitigated success of our Martian Rovers, planetary probes, space telescopes, and all the spacecraft that complete their primary mission and then some, is that their continued funding is preventing the funding of new missions. In the next few years, we might see the end of this Golden Age of Planetary Exploration as the current explorers wear out and few new missions fill their place.
Kepler and Cassini exceeded all expectations, lasting far beyond their primary missions.
In the meantime, will the Mars Opportunity and Lunar Reconnaissance Orbiter get congressional approval for discretionary spending to keep them running? I really hope so, as abandoning a fully-functional explorer outside our gravity well for political bickering would be such a painful waste of resources. It's still To Be Determined, so Americans, please write to your congress-critters to save them!
Image credits: NASA