Most organisms on earth have some utility. Even if they don't have an apparent utility, eliminating an animal from existence would wreak widespread disaster across many ecosystems. A reverse example is the introduction of just 6 wolves to Yellowstone Park in America which completely revitalized the entire ecosystem by putting predator prey systems in the proper balance.

Basically, if you remove one organism from an ecosystem, all the predator-prey systems are thrown off kilter and disaster can strike. Imagine a simple example: bunnies and wolves in an ecosystem. If there are no wolves, then the bunnies overpopulate the area which leads to too much grass being eaten, which leads to starvation for the bunnies. If there are too many wolves, they kill off all the bunnies, then starve. You need a proper balance between all organisms in the food chain.

I've sufficiently buried the lede, but my point is mosquitos have NO utility. They are a death scourge that only spreads disease. The most dangerous organism on the planet might be one of the only ones that we could safely eliminate without unintended consequences.