True, but if their top level requirement was to only gather data for 4 years, then they designed the solar panels to simply provide enough power to survive that 4 years. If it survives longer great! But due to their requirements more than likely not being beyond that, they don't need the extra weight.

Remember, they had to maximize overy ounce on the craft to meet the mass limitations. Otherwise they weren't gonnaake it to Mars. Any attempt to add parts to clean the solar panels means they'd have to downsize or outright remove some scientific payloads they brought with which would further limit the landers capabilities.

November 26th is their official 4 year mark since landing, so this dust storm may take it out prior to hitting that specific requirement.