The classic theory holds that there is a (single, stationary) hot spot under the slowly-moving central Pacific plate, and that the much more heavily-weathered (eroded) Hawaiian Islands towards Midway are the oldest and “coldest” and shortest.
Makes sense. But, if there is only one hot-spot, why is there a new island growing to the southeast of the Big Island (still underwater now) if the main volcanoes on the big island are still active?
It would appear that the hot-spot "source" can support multiple vents. Hualalai was recently active (1801), and Haleakala's (Maui) last eruption was in 1790. Mauna Loa is obviously still active -- this has been a relatively long hiatus for it.
Given that record, it's hard to even count Mauna Kea as extinct. Far be it for me to figure out the plumbing under the islands.