I'm pretty content with this. I loved Black and White's gym leaders, since they each played active roles in the progression of the story, but to this day it's hard for me to even remember all of the X and Y leaders. Only a couple of them ever left their gyms or had significant dialogue. It's hard for me to get invested in a character or location when the only interaction they offer is "fight the boss, then leave." I think the captains and kahunas have a lot more potential to participate in the story, and it seems they'll be interacting with the player in more ways than just standard battles.
It's hard to say whether or not this will be a permanent change for the series, though. The "Island Trials" concept is clearly very tied to Alola. We certainly won't be seeing the same exact system in any future generation. Not to mention, we see Gyms in the Pokemon world's equivalents of Japan, New York, and France - this suggests that Pokemon Leagues exist worldwide, across all kinds of regions. But I guess it's possible that new regions in the future could have their own unique challenge systems, like Alola does. If we do go back to having Gyms sometime in the future, they may need to be overhauled to keep players interested in them, now that Sun and Moon have strayed from the formula.