The original NASA estimates took into account the expected build-up of dust on the solar panels, which was expected to slowly diminish the electrical power generation of the rovers. This was expected to lead to their demise in a few months, as the panels were not equipped with any cleaning instruments for the panels, and non-working panels mean dead rovers.

However, the winds of Mars have proven to be highly reliable and unexpected cleansers of the panels, even un-doing the caking that was caused by the recent massive dust storms. This was simply not expected: too little is known about those winds, in terms of speed and frequency, for anyone to have a real estimate. Given continuous cleansing of the panels, the rovers have now performed beyond expectations - although they have had a number of smaller mechanical breakdowns and outright failures. I believe one of the rovers can only use several of its powered wheels and not some others, and one may have other drive problems. But NASA has been incredibly skillful in working around the problems, re-programming on the go where necessary, and just plain being cautious in how they handle the rovers.

So, again, they were not over designed, it was just that the original estimates forecast a demise who's cause has been abated. By Mars own doing, no less.