NASA definitely does overengineer things at the beginning, but it's worth noting that an incredible amount of work goes into keeping these things alive past the

NASA definitely does overengineer things at the beginning, but it's worth noting that an incredible amount of work goes into keeping these things alive past their end date. For example regularly updating the software to be more efficient so probes can keep functioning and communicating despite having less and less power and being further and further away. There's also lessons learned once a mission is in progress, like "if we move in this weird pattern we can shake some dust off the solar panels." Finally, a lot of these missions that continue long past the predicted end date do so with some limitations - maybe going forward a particular sensor is unavailable or certain maneuvers can't be done anymore - but there's still enough to justify keeping the mission going.