Show me a Mars rover that was still functioning but didn't get a mission extension so was just left to do nothing and decay. I doubt you will. If they are still working they are practically guaranteed a mission extension because the cost of continuing to operate a rover that you have already placed on mars and paid for is marginal relative to the mission. The most expensive part is designing and building the probe and then getting it there.
>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.
That's a massive exaggeration. I am not advocating for a contingency to every possible failure mode. I am pointing out that we clearly have one very common failure mode that can be solved and likely get us far more utility for little investment.
If you are going to compare it to Navy Seals, what I am suggesting is more akin to "Navy Seals can get shot. That's not good and these are highly trained specialists that don't grow on trees, we should probably give them body armor to give them a chance to survive being shot." And we do give them body armor. Yes it adds weight. Yes it costs more money. But more come home to complete more missions.
Yes a mechanical solution to remove dust from panels adds weight and cost but it can be the difference between an otherwise functional probe dying one day due to low power or dying years later once its electronics fail.