Except they didn't, hence the 90 day lifetime. Granted it was probably in there somewhere as a possibility.Fairly sure they did predict it would happen, but that little guy went 60 times past his original mission time. Even a best case was probably a year.
I would hope that with the next models they design panels that could rotate in such a way that dust wouldn't accumulate, but some will still do so from the static generated by the dust storms.
Even that idea has problems though.