You misread that entire thing. It's not that the team was bad, or that there was any significant turnover. Nor is Tim unable to transfer his knowledge, since that's not the point of what he does.
He's a sounding board. A sober voice asking the right questions when you've got your head down hyperfocused on a problem. Someone to talk to just to make sure you're on the right track, even though you were pretty sure anyways. The team would still perform without him, but he improves their quality and performance just by standing in for the duck and actually asking the right questions. It's a very subtle, very delicate job. Not everyone can do it. That's why they keep Tim on the team.