It only repels 50% of the bites though. Thus, those that bite, will survive and make babies. Some of them will be able to bite, others not.

Eventually, only those that can bite will survive.

I don't see how it can go the other way. Where eventually since only half of them bite, that ratio declines to zero. Can that happen with evolution?

I suppose there is a tipping point where if enough of the population can't find each other then they all die alone. Like rats in Alberta.

50% less bites does not imply only 1/2 of flies are biting. It's more likely that each fly bites only 1/2 as much.