Board Thread:Questions and Answers/@comment-30628894-20170502222018/@comment-13682699-20170504082108

IF (and a big IF cause Asgore lets no one find the SOULS) Sans were to gain that power, he would probably just take their SOUL and break the barrier himself with his infinite power. Even if the one SOUL resisted being used, the other SOULS' combined power is still far stronger and would allow him to use it anyways.

P.S. Sans isn't actually that strong. He may be one of the hardest bosses to face but that's taking into consideration his knowledge of you and your power and his very specialized skillset and general personality that is perfect for tilting you and making you ragequit. Most other monsters are generally stronger but don't have the same level of insight like he does.