The problem is that island sanctuaries, which could have been quite some extensive content, simply wasnt that, its extremely dull and repetetive, and features barely anything to do. Even for those who like gathering, the rewards arent even nearly worth it (effort to value conversion rate is absolutely horrible). If they involved content that required for example combat, and other special events, it could have worked. But it simply wasnt that.
The only way IS can be revived would be if it contains content for the DoW/DoM. Even if its just special boss fights like BLU has, where later levels unlock more of these fights, it can do quite a bit. As those fights could give special rewards which then in turn can interact elsewhere (relic weapon requirement, special ingredients for certain housing items, special glams, rewards that benefit IS again to unlock more, daily/weekly challenges). But that obviously requires a massive change (which they dont seem to want to do because of ffxvi). And in the end, its probably very similar to variant dungeons anyway.
And even then, the issue of IS is that its massively timegated instead of being actualy properly grindable content that is fun to do (instead of contant grinding for resources, things like fates are more dynamic). This timegating wouldnt be an issue if at least it didnt feel that artificial (fates can be a limitation as well, but have a more common spawn rate which allows people to do duties between them in case all of them were done).
I think IS is the reason we are currently facing a lack of more dynamic content. And because its too DoL oriented, its not something easily changed either.
And by now, even if they had an idea for such extensive content, im certain they will just wait for 7.1, as when 7.0 is released, all 6.x content is instantly becoming mostly obsolete as the new expansion is then where it is all at.