Considering how well the books have sold I think that's ok, that's the point, readers that enjoyed the books want the 'stamp' of the books as you phrase it. They're going to be very disappointed if it doesn't come through.
Ian openly campaigned for the role, I don't know what happened there, maybe we'll never know. He posted about it on Twitter constantly even so far as to auction off an autographed copy of Fifty Shades with his notes in the margins. I don't think Matt Bomer was ever truly interested, readers just wanted him because he looks the part of Christian. You can put down the books as 'literary crap' all you want but it doesn't change the fact that readers all over the world loved them, they stayed on the best seller list for a
record amount of time. All the insults in the world hurled at the books are a moot point, they kicked a$$ in sales. They were read for fun, no one was reading them looking for a literary masterpiece.
You can insist on being negative but why not try an open mind instead? You could be pleasantly surprised.