The sad thing is that Spirit could still be with us today too if things had played out differently. When Spirit got stuck a lot of their early attempts to get out so that they could get to a good wintering grounds were in vain. However, right near the end they came up with a clever way to "swim" the wheels through the sand and were nearly out when winter hit and they had to leave it in a poor location... where it failed to wake up the next spring, most likely due to excessively low internal temperatures.
Curiosity is great, but the cost of Curiosity-style rovers is just so high. When I think of all that could be done with the Mars 2020 budget (Curiosity-style clone).... ugh. I would have rathered they make incremental improvements to a Spirit / Opportunity style design than a Curiosity one. Maybe more / larger radiothermal heaters so that they're not as cold-sensitive and improved wheels and flash storage, for example. Get their price down to ~$350M USD per mission (from $410M/rover for Spirit & Opportunity) rather than 2,1 billion USD per mission (aka Mars 2020, down from $2,5M for Curiosity). Send a new pair for $700M with new sets of instruments to new areas, save $1,4 billion, and put, say, $800M toward a new Titan mission and $600M to a new Venus mission.
I just don't like how Mars keeps becoming more and more of a money pit that sucks the funds from exploration of every other part of the solar system.