Whether they're needed or not, scientists are already developing the means to make mosquitos go extinct because of the diseases they spread. No, really, this is a thing people are actively researching right now. Someone might enact this within our lifetimes. All it would take is one decision to let the genetically modified mosquitoes loose on the world, so they can breed with the originals and bring about their demise within x generations.
Of course, the opposite could also happen if someone out there is an absolute asshole and wants to release modified super mosquitos or it even just happens accidentally, I guess. Let's add that to the potential doomsday scenarios humanity could put itself through in the same category as stuff like lab made diseases spreading across the planet via infected individuals using airports and stuff.