I'm willing to bet that the immediate impact of this on the size of the fly population is negligible, so the trait is selected for in the short to medium term.

I'm willing to bet that the immediate impact of this on the size of the fly population is negligible, so the trait is selected for in the short to medium term. In the longer term, as the fungus spreads, it may be a victim of its own success. Fungi don't really understand sustainability.