You don't really understand the mechanics of the island falling, do you?
From what I've seen, the most commonly accepted theory on why the island falls is like this:
The island has a natural charge of +1 gravity, because of, well, gravity.
Jenka has a charge of -1, as she uses magic to make it float.
Ballos has a charge of +1, as he uses magic to make it fall.
The core then has a charge of -1, as it has some sort of mechanic to make the island float.
This all adds up to a charge of 0, making it rest immobile.
Once the Core is destroyed, the charge goes to +1. This makes the island fall.
You would assume that Jenka would not be making a difference here, as it's two forces pulling the island downwards at once against only one pushing it up. However, once the island hits terminal velocity, as it normally would, the other downwards force is doing effectively nothing. This means that Jenka can slow the descent considerably, perhaps even enough to prevent the island's complete destruction.