You can’t just slap 10% more without consequences. You now need to design a spacecraft to dissipate 10% more heat on all phases of the mission including when it

You can’t just slap 10% more without consequences. You now need to design a spacecraft to dissipate 10% more heat on all phases of the mission including when it’s well insulated sitting on a rocket ready for launch, for a mission that you want to last 20 years the initial heat load can be quite problematic. And picking a specific end date means giving up on the long tail of overly successful missions. Spirt was sending good data from May 1, 2009 to March 22, 2010 while stuck and NASA kept trying to contact it until May 24, 2011. https://en.wikipedia.org/wiki/Spirit_(rover) https://en.wikipedia.org/wiki/Spirit_(rover)