The worst man-made ecological disasters in history caused tens of thousands of deaths. We can say with certainty that killing off a few species of mosquitoes will not be worse.

You certaintyometer needs to be tuned. Around 129.000 people died in Hiroshima and Nagasaki and at least 200.000 people killed in Chernobyl until data. And this is a child's play compared with the >85 millions of humans killed by a worst ecological disaster. We can say with certainty that nobody thought in ancient times that those small and clean rodents, living peacefully in the grasslands and steppes of Central Asia were something to worry. Rats in Europe were not unlike the good species of mosquito if we think about it, with their own diseases and new random relationships...

You're suggesting that because it's never happened before (that we know of), we can say with _certainty_ it won't happen in the future? Can we go back 100 years and say that about climate change please?

1 million is a really big number, which is what makes the certainty easy. That's even bigger than projected deaths from climate change.

Under what hypothetical scenario would killing off mosquitos cause two million (human?) deaths per year?

Easy. Untold side effect of MTBe caused infertility in 50% of cases in second generation of black women in treated regions. Entire country was depopulated in 25 years. Like that research article from 2056?

What if the diseases that depend on mosquitoes to spread mutate/evolve themselves to spread via air...

You know, "life finds a way" and all that..

Which is why people are studying the issue, writing papers and popular articles, instead of just doing it.

Despite the link-bait headline, no one is talking about killing off all mosquitos.

And yes, gene drives can be reversed.