I think in the past all of Nolan's leads have worked out exceptionally well given the characters they were chosen for. I think DiCaprio was fantastic as Cobb. I don't think I could see Bale, who would have probably given Cobb too much of a chilly demeanor in my opinion, would have fared well in the role and I don't think Jackman has the right sense of intense desperation that somehow DiCaprio was able to pull off. I don't know. I just think DiCaprio was properly suited for Cobb.
However, given Cillian Murphy's surprisingly effectual and emotional performance as Fischer, it would have been interesting to see him as Cobb but I think DiCaprio had the right acting prowess and of course star power to pull off Cobb, and he did.