I find that often those “gaps” are created by management not pushing some people as much as they push the gap fillers. The gap fillers might be better employees, but why should they have to put up with an underperforming person?

Basically the one doing great gets added a bunch of work while the one that’s “not good at that” doesn’t get pushed to learn it because management doesn’t have any faith in them. Then the good employee leaves and everyone is like Aw man whyyyy.

I deal with this by refusing to accept responsibility without authority. If you want me to be responsible for a team's success, then give me the authority to actually make the team successful and pay me accordingly. Otherwise, it ain't my problem.