The hard part is defining what "just enough to meet the 90 day goal" is. The rovers were things that had to be built to survive a flight to Mars, go though the

The hard part is defining what "just enough to meet the 90 day goal" is. The rovers were things that had to be built to survive a flight to Mars, go though the atmosphere, land by bouncing on some airbags, and then go drive around in a very hostile environment - which needs a pretty sturdy robot. Over-engineering the robot itself is also a form of insurance. Launching a rocket is hard. Landing a lander with airbags is tricky and very unpredictable. The time from finishing to build a robot and it landing on Mars is a really long time. Funding to pay for all this comes once or twice in a generation. Given all those possible points of failure in things that have nothing to do with the rover itself, why not engineer the rovers to be as resilient as possible to try and make up for some of the possible risks that could happen elsewhere in the project? Put another way - if you're building a race car and the body costs $1000, why not spend $20 (as opposed to $5) on the engine to make sure that of all the things that could break, the engine won't be it.