HA is a pretty responsible monopolist. I'm sure they're making solid money on interisland flying, but when I've compared interisland flights to similarly short routes elesewhere, they don't look out of line. If anything, they may be relatively higher for advance purchase and lower for close in, which makes a certain amount of sense with HA's mix of local business, local personal, and tourists. Southwest may skim off some traffic in the mid-day tourist bulge with interisland legs sandwiched between mainland legs, but I'll be pretty surprised if they try to compete seriously with HA's schedule.