Well theoretically we have all six elemental crystals so we always could have summoned the one that was still missing. I am not sure if its correct but each WoL probably will take away some of her power. The crystals seems to help us against the Ascians but they have her aether in it. If we are able to give back her power by giving her our crystals we are probably also taking away her power if we get a crystal. We dont even know if the other worlds had no WoL. Maybe these WoL just lost to the Ascians? We are also her last trump card, so maybe she just cant "create" more WoL without using the last bit of her power.
Its also the question if everyone that is choosen as a WoL, really goes along the path. She never truly forced us to help her so maybe some choose not do to something?