I think we’ve already talked about what if all the world’s disease-carrying mosquitoes (and only those species) were eradicated. One of the ecological disasters that might precipitate could be the burgeoning of the human population, especially in regions that could least support that.
Okay, now: What about the common housefly? Do they have any ecological value? If we eradicated every last one of them, what would happen? Would the sky fall on our heads?
Houseflys carry diseases. There are a lot of similar insects that probably would end up replacing them and carrying the same diseases. I think the housefly has developed some resistance to common pesticides, so other species may be a little easier to control.