A healthy ecosystem in Singapore will be chock-full of mosquitoes and no humans.

The island started out as a swamp.

And, ironically, other insects (for example, dragonflies) which are also killed by the fogging. What's more, the predatory insects usually have a much longer life-cycle, so after a fogging the mosquitos will come back long before their predators. Thus, once you start fogging, you have to keep doing it or you'll end up with more mosquitos than before.

I've actually seen this happen... ok, my observation is purely anecdotal and has no scientific validity, but the effect I observed was pretty dramatic. Where I live in South America dengue is also endemic, and during some particularly bad Dengue outbreaks a few years ago the city government did some extensive one-shot fogging. Immediately afterward there were practically no mosquitos at all for a few weeks and reduced numbers for some months. But thereafter for at least 6 months to a year everybody complained that the mosquitos were worse than ever (mostly without making any connection to the insecticide fogging, they just complained the way people complain about the weather).

What makes you think a "healthy" ecosystem isn't deadly for humans? I mean, malaria, yellow fever, dengue fever are all natural parts of the ecosystem.