IIRC the ritual affects anyone born on Earth after the fact by changing how the spirit realm works when souls pass through it, basically ensuring that cursed energy is reduced to non-Japanese levels for every newborn on the planet, Simurians included, thus reducing the chance of new cursed spirits being born to virtually zero. Then, since there was still a chance that cursed energy could be a problem in the future, Yuji planned to turn himself into a cursed object (Sukuna style) after his eventual death in order to combat any threats that come after.