The heir and a revenant going through the border on the edge of the Triangle brings down the Triangle so the revenants can leave.

[So yes, it is a solution to their little problem.]

But it also means it lets in everything else waiting on the outside, trying to get to the Garden. So that means a lot of monsters coming in, and if the glimpse we got when Willa tried to do it a couple of years ago is anything to go on, they're big nasty ones.