The fact that Shield success doesn't guarantee Test success is beside the point, and the sort of argument people who don't watch the Shield resort to. 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.
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.