They started stacking stones when they got stuck on the island, and she symbolically stopped with the seashell when they were free. It is the intended final addition. Kinda makes it an informative monument, they have a sort of physical record of how many generations were stuck on the island.

? But there's not gonna be another, that's like a bit plot point

Isn't that exactly the whole point though?

4d move. She is always on top because she's the leader that changed the game.

Bro they left that island tf wrong with you

Well when the villagers inevitably forced to return to the island, I imagine the next rock to go up will effectively crush the shell. Just like crushing her legacy.

Rock crush shell