Joachim said his alternate proposal was that the children lose them at age 25. He thought it was being considered. As he said "Athena is 11 in January". Clearly the timescale ended up being vastly different.
I find even that proposal stupid . I would not have done anything to the current grandchildren (both Joachim's and Frederik's kids) and then restrict the prince/ss title to the children of the heir (Christian's future children ) while the rest get's the count/ess title