Comments from feedback form - "If Opportunity has been on Mar..."
If Opportunity has been on Mars since 2004, and it's operational lifespan ended in April 2004, then did NASA really expect the rover to not last that long?
Well, they set a "prime" high priority mission for the rover to accomplish, with the optimistic intention to carry out further exploration beyond the designated lifespan. It's more technical than anything and most spacecraft function past their lifespan, such as the Voyager program probes, which continue to function after their launch in the 70's.