I'd have liked to see either Vaughn or McCallum as Waverly and maybe if this had been done ten/fifteen years ago that would have been likely but they are both, sadly, getting on a bit now.
Leo G. Carroll was 72 when he debuted as Waverly. Robert Vaughn is currently 80 and David McCallum is 79. So they're both around the right age -- certainly closer to it than Hugh Grant is. I wouldn't mind seeing McCallum in the role.
Then again, I suppose the studio is thinking franchise, so they want a Waverly that's likely to be around for a few sequels.