Well it seems like the Mars landers/rovers are managing to exceed design life enough that something to clean panels would probably be coming down the line eventually, and extra mass allowable certainly would make that easier.

Assuming the cost estimates are actually close, that will definitely help. Although in some places (see high rad environment near Europa) you still wouldn't want COTS hardware. And some things like RTGs are probably never going to be available COTS.

The martian atmosphere has a very low density, so you can't just use a normal compressor. And any compressor you bring is extra mass and volume you can't use for science. And you have to think about how a compressor running would impact sensitive instruments.

Not mentioned, but using compressed air means either bringing said compressed air, or bring a compressor. Both require control mechanisms, and additional mass and volume that can't be used for science.

Think fine dust buildup as opposed to normal sand you might find at a beach. Also the physical mechanism needs to change to be able to rotate the 180 (or even just 90 degrees) For Insight, the shape of the panels means I'm not actually sure there is sufficient ground clearance to rotate the panels.