I don't think the doomsday argument is a bad argument mathematically. It's just completely useless, like predicting whether the sun will rise tomorrow using laplace's rule of succession. We have vast amounts of information that has some bearing one way or another on the likelyhood of the end of the world happening at any particular time. It's absurd to throw all that away. As such, dismissal seems completely reasonable to me. I really don't think there is anything to be learned by calculating the expected number of total people ever to exist using nothing but a uniform prior.