It’s several things I think pulling Harry in several different directions to create a balance, the Mantle, Soulfire, Maggie and his family, his Elemental Nature, Demonreach it’s the pentagram all over again, because he is Starborn. Knock out one or more and you don’t have Harry. Ethnui knew this so she attacked his who whole father/child issues.
Alt Harry in MM only has his elemental nature, unbalanced.
From Battle Ground by Mab p53-54.
“Immortality offers a significant advantage, but it is no substitute for intelligence. Remember that, young wizard.” Ebenezar scowled and opened his mouth. “Should it for some bizarre reason ever be necessary,” Mab said smoothly, before he could speak.”
Immortality is on the table for Harry, but not for everyone see the Darkhallow. A successful Darkhallow requires a Starborn? But the Darkhallow only magnifies that which is already there.
This makes me think that it is easier to gain immortality through a Mantle rather than through any other means.