2018 saw an absolute record number of seats/capacity between Hawai‘i/Mainland with the major players in the market like HA & AS reporting soft demand/depressed yields in the last quarter.
It will be interesting to see how SW prices & markets their fares to Hawai‘i since they do not offer "un bundled" pricing or distribute through the OTA's.
My 2 cents & YMMV is that I think SW will at best affect prices at the margins and the real story will be who cries uncle and reduces capacity first during 2019/20 as the economy and demand retreat from record levels. As most mainland companies learn ... (successfully) entering any market sector in Hawai‘i requires much more patience, capital and long term commitment/horizon than they first anticipated.
For those that are "data" hungry for Hawai‘i Travel Market statistics check out:
https://www.hawaiitourismauthority.o...or-statistics/
Data | UHERO
Other factors affecting SW ability to put a lot of capacity into inter-island routes are:
Engine cooling issues - short inter island high frequency flights. A good thread at:
https://www.airliners.net/forum/viewtopic.php?t=1408499
Available Gates can be in short supply during peak travel times.