I think the main point is that nature is in a dynamic equilibrium that's built up over eons. We disrupt that equilibrium unintentionally by things like development, but well intended disruptions can have just as negative effects. The typical example would be something like removing a predator (or even a disease) from an area which results in a population explosion of its former prey which results in increased pressures on what that prey eat and so on all the way down the food chain.

And it's not just hypothetical - for instance gray wolves were largely eliminated from many areas with catastrophic consequences. They're now being reintroduced in many places and you get interesting effects like it turning out that gray wolves were effectively helping keeping a healthy beaver population, which is particularly interesting given that beavers are prey for wolves! [1] It's just a really interesting interbalance, and changing one thing can have consequences that are practically impossible to predict.

This is the reason I'm not a fan of the idea of eliminating even mosquitoes at large. Unforeseen consequences are very much a thing, and those consequences don't inherently become 'seen' because of a study or two.

[1] - https://www.yellowstonepark.com/things-to-do/wildlife/wolf-r...