Something to think about, possibly good or bad. If these rovers hadn't had such longevity, would have had other rovers on Mars in the meantime? If so, would they have been more capable? Or have these rovers represented such tremendously cost effective experiments that we're getting what we would have gotten with other rovers, but saved on launch and opportunity costs (awaiting transit to Mars, possible mission failures, etc.)?
No, there would not have been other rovers on Mars in the meantime. There were no rovers in development between MER (Spirit and Opportunity) and MSL (which is now en route to Mars).
There's a lot riding on MSL landing safely, because there's nothing solid after MSL right now. There used to be a 2018 rover, but it was recently cancelled.
