Would firing Seth Rogan have saved the movie?

Because half hour or so of the movie was great fun, as the two geeky Brits went to Comic-Con and hit the road into alien abduction country. But then it goes sharply downhill and becomes a drag.

So, why exactly does the movie fail as it goes along? is it the annoying and deadly presence of Seth Rogen, or does bringing religion into a comedy kill all the laughs?