Or this could be one of those situations where the content is being received differently by the NA player compared to the JP player base.

Evidently they're thrilled with the mammets and having a good time. I'm having a good time with my island but think it would be even better without the mammets.

Players are not a hive mind. They're looking for different things. If you only find combat content entertaining, then IS clearly is not intended for you since it lacks combat. Of course you will end up bored. Fortunately, the game has plenty of combat content and the new Variant/Criterion dungeons are coming in a few weeks so you'll be getting new content (hopefully) more to your liking.

IS works for those who appreciate it for what it is instead of trying to hold it to standards appropriate for a standalone game despite the repeated warnings we were given.