Yeah, sure, all that is fair. Nonetheless, that is the Shield, and really doesn't prove that much - just look at Bancroft.
The fact that Shield success doesn't guarantee Test success is beside the point, and is basically how people who don't watch the Shield justify ignoring the Shield.
The point is that Marsh failed at Test level because he lacked temperament and struggled off the back foot, and anyone who'd seen him play recently would have known that he has improved both those things exponentially.
You can't blame people for being disappointed he was selected before Maxwell though. Maxwell has been in better form and was one of our best batsmen in India. Reasonable for people to suggest Marsh was not next in line.
Maxwell was never competing against Marsh, because Maxwell is not a fourth pace option. If they didn't want four pacemen, Marsh wouldn't have been selected.