If we take just the good parts of this and compare it with just the bad parts for that, we can go really far before crashing against reality at incredible speed. But cherry picking is a lot of fun and two can play this game:
Can you assure that saving this 20,000 humans will not mean one million humans starving and dying later when their main source of proteins (fisheries) will vanish forever by lack of mosquitoes and introduction of mosquito fishes (those pesky fish egg eaters)?. Wouldn't choose to save one million babies, you? Wow, that's evil. Think in the babies, people!
You should probably stop doing anything then, because you can't prove you getting out of bed in the morning won't lead to the destruction of society. You can't assure me it won't!
Even us filtering water for drinking has downstream effects which may ultimately cause harm which can't be entirely unproven. I guess we should just stop drinking clean water!
There are tons of things we just ultimately choose to do or allow to happen which can have potential issues we may never even know are happening until its way too late. Ruminating on maybes we have no way to even measure or know or simulate isn't exactly useful. What are the likelihoods of those outcomes? Pretty dang small based on our current knowledge? What are the likliehoods of tons of people dying or being seriously injured from these diseases? Extremely high?
Can you assure that saving this 20,000 humans will not mean one million humans starving and dying later when their main source of proteins (fisheries) will vanish forever by lack of mosquitoes and introduction of mosquito fishes (those pesky fish egg eaters)?. Wouldn't choose to save one million babies, you? Wow, that's evil. Think in the babies, people!