> The ecosystem will adapt.
Sure, but as with climate change, you may not like the result. I guess there's a fair chance that it won't be "just like now, but no mosquito bites". Mosquito larvae are a big source of food for others, and extensively prey on larvae of other species, keeping those in check. Adult mosquitos are important pollinators, too, and as we're currently discovering with bees, taking out an important pollinator has big consequences and while the ecosystems will adapt, it may well end up shedding very desirable species entirely, ecosystems take a long time to bounce back, and may be a lot less appealing to us afterwards.
Frogs and fish might suffer, too, they'll have to make do with a lot less food when you take mosquitos out, and what takes their niche may not be as palatable. You push these below a sustainable population size in an area, you probably lose them entirely, and their function of keeping insect larvae and adult insects in check. Since we'd like to enjoy mosquito-free summers ourselves, and soon, the time scales we'd typically do such interventions on is way too short for much adaptation in vertebrates to take place, generation times even of frogs and small fish are too long for that. You may end up with ponds that are way emptier and more silent, and with lots more creepy crawlies and possibly a long-term balance that has, say, way more rats, roaches, etc.