What was Mankar going to gain out of having Dagon destroy Mundus?
By having Mehrunes Dagon became the kind Leaper Demon King again by destroying Mundus, which will also free the mer from the oppression of Mundus so they can go back to the Dawn. The general altmer population also carry with them a version of this belief, in that Mundus is nothing more than a prison Shor/Lorkhan/Shezzar created in order to weaken everyone. What the altmer have been doing for the past 5000ish years is to reverse this by extreme selective breeding (they need to be as god-like as possible), and a strong desire to go back, so they can be the "awesome kick ass gods" they were before. The exception to this is the dunmer, and Vivec even has written that he sides with Shor's intent to make it so that mortals can ascend higher than the gods.
(most) Men, on the other hand, view Shor/Shezzar/Lorkhan has a savior. His creation of Mundus was an act of divine mercy, so that the mortals are free of predictability, and pretty much what Vivec said. The exception is the redguard, whose views are merish (Mundus = prison), but do not view Akatosh and Shor in a good light (the both are devouring serpents with an insatiable hunger).