In my opinion, for what little it's worth, a driver who cannot pass a backmarker without having problems is not such a good driver. Even on a one groove track since all of the tracks are more than one car wide. Yeah it can be tough, but in the Cup Series you're supposed to be tough and capable.
It doesn't take being off on setup by very much, or much minor damage, to slow you enough to be at the back these days. Cup races are long, and good teams can usually figure out how to get their cars back up to speed over the course of a long race. Let them have the opportunity to do so.
NASCAR does do a pretty good job of issuing the move-over flag, and drivers who expect to earn any respect are pretty good at heeding it. But the slower drivers and teams need the laps in order to learn how to become fast, so don't curtail that effort just because they might get in the way of a hero driver. It's part of racing. The really slow or dangerous drivers will get black flagged.