They eat other things as well. Supposedly mosquitoes are pretty much the only living thing that's completely useless, and can be eradicated with no ill effects.

They don't really control mosquito populations that much. Dragon flies prefer fresh permanent bodies of water (I think usually flowing too), whereas mosquito larvae inhabit stagnant pools.

Only like 3 species out of the something of 3500 mosquito species drink human blood/ carry diesease this way, and they only do this during the females pregnancy not as a primary food source.

And of those 3 species, I think one makes up the majority of mosquitos that are found worldwide and this is the one that would be targeted

https://askabiologist.asu.edu/mosquito-species