I agree, but Daedra aren't gods, they are the things they embody.
That's how gods were viewed in the old days. As in, when the sun moved across the sky, the Greeks would say he (Helios) moves, not it moves.
If there were no Destruction, Change, and Ambition, there would be no Dagon. If there were no Dagon, there would be no Destruction, Change, and Ambition. They all play hand-in-hand. So you really can't change him and make him want to spare the world. They provide these concepts (like Destruction, Ambition, etc.) with a physical form.
Sorry for picking apart here, but just like Dagon represented the potential destruction of Nirn and the mortals, he also represented the destruction of the Aylieds that let the empire became what it is in the first place. If he is, indeed, the embodiment of all destruction.
So destruction isn't always bad... change and ambition aren't always bad... and if there's something good in it for him, I don't see why he can't be prayed to to halt destruction and maybe even focus that destruction elsewhere.
If you're worshiping Dagon, you're worshiping the existence of Destruction, Ambition, and Revolution. Just like if you worship Sheogorath, you're worshiping existence Madness. The Princes just give it a tangible form. So the worshipers of Dagon would know what they're getting themselves into, and Mankar Camoran (He may actually be part Bosmer and part something else) did a lot of brainwashing... he mighta been crazy himself.
As I said above, I'm a big nerd about religion and mythology and I can tell you this is exactly how the old gods work. People wouldn't worship destruction and change because they are self destructive, they would worship it because of its potential benefits.
And if that destruction they were worshipping starts to destroy them instead of their enemies, they will become confused and pray to it and sacrifice to it to try and appease it.
In the case of Mankor Camoran, this is a man who twists religion and preaches lies in order to brainwash people. Maybe he made a deal with Dagon, but that doesn't mean someone less evil could make a more tempting deal with Dagon and convince him to destroy Mankor Camoran instead of destroying Nirn.
Why not? Destruction is destruction.
EDIT: I thought of a better way to phrase my thoughts. When you weed and plough a field or dig a foundation for a building, you are destroying what nature put there in order to change that plot of land into something that benefits you.
So, Mehrunes Dagon (Destruction and Change) plays a vital role in the harvest and the building of an empire and, indeed, everyday life. Mold destroys your wall so it has room to grow, then you destroy mold so you can restore your wall. Dagon represents the endless cycle of ridding the old to make room for the new and to try and stop destruction is as foolish as to try and stop the rotating of the planet.
If destruction and change are no more then, yes, Nirn will last forever, but also, no one will age and trees will forever grow and take over the cities and there will be no means to clear the land for civilization ever again.
It would be better to pray to destruction so that it, once again, works in your favor, at least for a little longer. Fighting destruction, like the Blades did, is a foolish, futile venture. All they did was hold off the inevitable and ensure that it is extra wrathful the next time around.