To me at least, the scenario of moving from Montana to Washington, and leaving cars registered to Montana, seems less wrong than the commonly abused loophole.

As I understand, a fairly large number of people register cars in Montana while never living in the state.

Edit: Just to add, maybe the scenario you described should be cracked down on, but it feels like less purposeful avoiding/evading a tax. My opinion is that could come afterwards, with more warning given to people.