WN has a blackhole in the Southeast that a single gate won't fix. They'd put competition on FL to MDW if that destination is chosen. FL currently flies a 737 on that route. It looked like DL had a flight ATL-MDW when I was there yesterday, but DL has better presence at ORD also.
Could they do more than just connect to hubs? I know WN is more distributed than other airlines, but that would take more gates. Maybe ATL-BWI? That would hurt FL more too.
I think FL stands to lose more in this than DL.
Since CLT had no Frontier either I guess we're still safe.