I think they are capable of holding the White Frost back for a time, but not infinitely, that is why they need Ciri.And as Avallac'h says in the book spoiler above, they want to use Ciri or Ciri's child to open a portal so they can leave their doomed world. And that is also what Eredin seems to want to do in TW3, although he would likely bring chaos to those worlds. But, Avallac'h seems to want to stop it altogether in TW3. I don't really know how Ciri's power is supposed to stop it, though. Sapkowski is rather vague, deliberately I suppose, regarding the White Frost and Ithlinne's prophecy. Either, people believe it will happen or that it is just "elven hogwash".