Let's guess: the gentleman is very capable, but most candidates are not. Any potential replacement hire would need a reference from a colleague whom you trust

Let's guess: the gentleman is very capable, but most candidates are not. Any potential replacement hire would need a reference from a colleague whom you trust just because the pool is so awful, and even then the new guy would need extensive training to be up to snuff. Old guy can't leave because at any potential new place the people making the hiring decision don't trust your reference because they don't know you. Sounds all very familiar.