Okay, worldwide there are over 3,500 species of mosquitoes. Some of them carry human pathogens. These disease-carrying species are the targets in some of these efforts, like in eliminating, or making disease-resistant, species of malaria-carrying mosquitoes. Invasive, non-native species are the targets in other control programs. The accidental introduction of non-native species like the Aedes aegypti in California is not ‘natural’ so their eradication is not necessarily any less desirable. If exhaustive and scientifically-defensible efforts are made to combat collateral consequences, then a case can be made for these programs. There are historically spectacular and well-documented failures in ‘natural’ control interventions (e.g., the cane toad et al; which was very successful in some areas but far less so in others). Because of these real and serious issues, I will hazard that more rigorous laboratory and field studies precede the current releases of biological controls into the wild. Not that mistakes can’t be made, just that they are less likely and arguably less detrimental than doing nothing in the modern context.
There are untold numbers of extinctions happening all around us every year through totally banal (and ignorant) human activity such as normal ‘development’, and exotic species are dumped into every wetland, forest, and desert every day by suburban folks emptying out their aquariums into local rivers and swamps or gardeners letting their random plants run wild. By contrast I’m not super bothered by a well-studied, concerted, and targeted biological control program like this.