Even if the penis is extremely large, I don't think it can replace dilation entirely. I haven't had surgery yet, so I don't know, but don't they say you have to dilate every day for 1 to 2 hours or something? Even if you do have daily sexual intercourse, does the penis stay inside for that long? I mean, even if you do make love for an hour every day, does the penis stay inside the whole time? And does it go all the way to the bottom for all that time? I don't have any sexual experience, but this doesn't sound like a very normal way to do it.
Moreover, thickness-wise, I'm really not sure there are many men that can match a dilator. And since (I think) the most common problem with lack of dilation after surgery is a loss of width at the opening (I think a more stiff, more narrow ring tends to form at the entrance), it wouldn't really protect from that.