Here’s a team. Someone leaves. We hire a replacement. We get lucky and manage to find someone more skilled. Looks like we’re better off?
Really when someone leaves we lose all the relationships they had with the rest of the team as well. The team is a diminished more like 40% than the apparent 20% by their loss. It takes longer to rebuild the team than is apparent. Relationships take time.
It’s worse than that. The team probably wasn’t maximally-connected to start with. And it’s not just the interpersonal relationships that matter but the knowledge of tech and domain. A departure can break teams apart and organisational knowledge needs to be rebuilt.
Your organisation probably has multiple teams. Someone leaving your team reduces its connectedness to the rest of the organisation. Increasing the time to recover even with a swift new hire.
Internal mobility is less of a hit to the team’s connectedness due to pre-existing relationships. It also increases the whole organisations resilience by establishing more inter-team relationships.