I'm really not sure why there's any concern. Miller's books (and art, and style), while they might've contributed to the redefinition of DC in the late 80s in a big way, have always been fringe, always been hyper-stylistic, but more importantly, always been sort of a place where he can just do what he wants, tell the story he wants, and have the freedom to do it his way. That Superman drawing is just that. My favorite ever? Nah. But that's why I've always been keen on them giving him his own Dark Knight Universe to play in.
I mean granted, there's certainly a difference between that Superman and the guy you see in DKR, you know, just keeping it in the Millerverse, but hey, not only does art get harder for older guys, but the emphasis on style and dramatic sketchiness versus a more classical portrayal changes. Shit changes, basically. Denys Cowan is often a lot looser than he was during say, The Question, but I still love everything he does, even that weird unnecessary Joker Origin retelling where Joker had too many teeth.
I'm eager to see what he does. I'm not the world's biggest Miller fan, I find a lot of his work problematic, but it's always worth picking apart and critiquing for one reason or another. And look how rad The Atom looks!