[SPOILERS] immortality of the sylvari
There is no way of telling if the Pale Tree actually has this power. One could argue that a large chunk of Mordremoth’s power comes from the consumption of Ley energy, something the Pale Tree might not have access to.
But even so, as she considers the Sylvari to be her children, not minions, it would be pretty unethical of her to just replace them with a newer version. It sort of drains the emotional attachment.
Joy to the world, ignorance is bliss