Sekiro has had the power of ressurection since the Hirata Estate incident. Genichiro chops off Sekiro’s arm with a regular sword (this is the entire basis of the argument, so if I’m wrong call me out please; I think he gets the other mortal blade during the course of the game but I could be wrong) and allows us to do all this awesome stuff with the prosthetic and have a blast. However, the mortal blade(s) are deemed the only things able to kill an immortal.

Even if the immortal doesn’t initially die, anybody could pretty much cut an immortal into a bunch of stubs who can never move or function. Makes the mortal blade kinda useless except for its use against the Divine Dragon and Purification Ending Spoilers: Killing the oathbound immortal to break the Divine Heir mortal again.