Your belief that the fragile ecosystems would have self-destructed long ago is based on the belief that the ecosystem has always been fragile.
The truth is that the ecosystem USED to be dynamically stable, 100 years ago.
But then a top level predator came in and started screwing around with it. Namely Man. We divert water, and the dynamic ecosystem compensates. We kill the other predators, and it compensates. We fill the air with too much carbon di-oxide and it compensates. We pump poisons into and it compensates more.
But after all this stress, it has reached it's compensation capacity. Now it has BECOME fragile. It wasn't fragile 200, 100, or even 20 years ago.
The reason it didn't self-destruct long ago is that it was strong 100 years ago, but isn't anymore.
If you screw around with anything long enough, eventually it becomes fragile and tiny little change will push the whole thing over.
I am not saying that the mosquito will be that change. For all I know, we can do another 50 things before we push it over the edge. But I am saying that your argument is itself heavily flawed and does not hold water.