Complex sentence: Tolkien emphasized that the restorative sojourn of the Ring-bearers in the Undying Lands would not have been permanent; as mortals, the Gift of Men would eventually lead them to die and leave the world of Eä.
Split into simpler sentences: Tolkien emphasized that the restorative sojourn of the Ring-bearers in the Undying Lands would not have been permanent. As mortals, subject to the Gift of Men, they would eventually die and leave the world of Eä.