He honestly expected them to last 120 to 180 days, tops. They did not see the
dust storms blowing the dust off the solar panels for one thing. Of course the
rovers, just like other space probes such as Pioneer 10 and 11 and Voyager 1 and 2,
were designed to survive in very harsh conditions, so their longevity is just a
"natural" by-product of lasting this long.

But it is safe to say that no one on the MER team ever thought they would keep
going this long, especially in a mobile state still returning data. I remember when they
hoped to have Spirit make it to the top of one of the Columbia Hills before expiring.

I wonder how long MSL will last once it actually gets to Mars?