The daughters will not need tiaras in the future, in addition Elena's children have the Marichalar tiara at their disposal, and Sofía has a large collection of jewels, including some inherited from Frederica and the Niarchos, which she can leave for her daughters and granddaughters.
The shell tiara is a delicate piece, that's probably why they don't use it much, but it has always belonged to the main branch, and I don't think it will come out of there. The Prussian tiara will surely be for Leonor to follow the tradition of wearing it at her wedding.