I think foiling may kill formula perhaps "casual" longboard racing/sailing. But at most places I sail, formula boards and longboards always were a rare sight, so that's not much of a kill.
Stretchy said..
Rat what foiling has killed for me is there's no one to sail with at the local now.
Funny, foiling has had the opposite effect at my local. in 10-15 knots, there's often more company on the water now than before, with 3-4 other foilers out at times. Just a year ago, seeing more than 1 or 2 windsurfers out happened maybe once or twice a summer. That should increase in the future as more windsurfers get foils; we're slow adapters here. Most of the windsurfers who have a foil have dabbled in multiple aspects of the sport before (waves, freestyle, slalom, speed, SUPs, ...) and just added one more way to have fun on the water.