Alaska, United,American,Delta. Alaska is the youngest carrier and they've been around for (10)years now and neither carriers have even implied interisland service? They must have done their homework and decided that it would be economically unfeasible.
Which brings up the question; why would anyone think that SouthWest would venture into a market which apparently other larger senior carriers would not??? The only thing I see SW doing are direct mainland to Honolulu, also to Maui/Kauai/Big Island-Kona/Hilo. and even with that being said, a full capacity going back, otherwise it's a money lo$er.