If you look at the title, this is "stuff for the rich." It won't benefit everyone, obviously. Just a select few. Coban has been saying that he's looking into things for the rich, since mansions/skyscrapers get quite boring (for me, anyways).
And I don't know how many times I have to say it. There will be no plots regioned on the island. There will be a set. Selling. Price. This includes forsale and resell. There will be no profit. Nooooo profit. It doesn't matter how valuable it is, you can only resell it for a certain price. No profit. There would be a set selling price.
I just don't see how this will devalue other islands/"ruin the economy." It makes no sense. These islands will make no profit and will be something for the rich to buy and build their dream estates on. It's only a handful of plots, maybe only 5 people could afford one right now? And a lot of people wouldn't even buy these, as there would be no profit and no regioned plots on the island. It's a feature that only the rich can have access to.