At the end of the day the best way to ensure mission success is to put the mission first, not future missions, not "nice to haves", put every dollar you have into making the prescribed mission as perfect as possible. You're suggesting the equivalent of "Navy SEALs sometimes get caught in larger than expected firefights, so each man should lug an extra 1000 rounds of spare ammo into combat just in case". And they could, but that would slow them down and endanger the immediate mission, even if only slightly.
The opportunity cost of that million dollars could be that testing is slightly less extensive and that a minor flaw goes undetected, or other work is rushed because the engineers were busy getting the wipers to work properly. It could also be that there's no interesting science to do in that area even with an extended mission, so the mission extension turns out to be completely worthless.
It's a different mindset than most engineers are used to, because we're used to users having vague desires and needs that even when well defined are somewhat ambiguous, so we plan generic solutions for a bunch of different edge cases and assume things will be used in ways other than what they're intended. And we all like building durable stuff that lasts the test of time. But when you're building a missile you don't care about minor memory leaks, and these probes are essentially science-missiles.