The backbone of this argument is that maintenance costs due to zebra/quagga mussels would be less than what we're paying now for closures/inspections. Which most people and most case studies I've read don't seem to support. It's not being overlooked either.

I don't necessarily think you're wrong about letting nature take its course, but humans crunching the numbers are part of nature taking its course. And at the moment, the numbers support keeping them out :-(