I'm not sure Leo wants to move to the demon world, so it might be it for him the moment Echidna acquires the philosopher's stone. But who knows. Maybe he would see things to the very end.
I'm thinking the philosopher's stone is probably a lie (and maybe leo's core). Leo and friends probably got tired of explaining their origins to an increasingly decayed civilization over the years and started saying "I just used a philosopher's stone to do it". Over the years the rumors grew and reached the demon world.
Erm, philosopher's stone explanaition fit MC like a glove? He was "kept at Holy Land", he is powerful, and can "grant any wish"... What if philosopther's stone IS MC?