Everyone is replaceable. Our favorite athlete, game show host, or pastor will eventually move on. Even with Jesus’s 12 disciples, they were replaceable.
After Jesus ascended back to heaven, they returned to Jerusalem to pray. This also included Mary, Jesus’s brothers, and some women. There were about 120 people in total. In this group, Peter suggests that they find a replacement for Judas. The two that are voted on are Joseph (called Barsabbas - also called Justus) and Matthias.