By your logic having someone 2x as sucessful at sales is risky and you should let that person go.
Builds good will, makes people happy, and secures your ownership of a product that has added tremendous value to your company. That's a win all around.
I think you grossly underestimate how hard it is to find one person, much less two or three people, who can and will actually do this task diligently.
Your best hope is that the PM gets bored and wants to move on, and you let them hand pick their successor(s).
I’m going to paste from another reply I made in this thread:
“Knowledge documentation and transfer in reality is hard. Very few people read docs, even when they are told to and those docs are critical to their job and the business as a whole. If you do it face-to-face, it’s almost always the case that the person learning politely listens and is already making a mental list of changes (often catastrophic) that they are going to make.”
I will add that hiring competent people is hard if the company is focused on keeping compensation relatively low (as in this example) rather than rewarding their champions.
I realize that as an org gets larger, pay bands have to be a thing because mediocre-but-functional people (i.e., most “good” employees) get butthurt when a champion makes more than they do. That said, I think that many SMBs can have substantial salary variation within the employees, have those variations be reasonable, and frame it in a way that makes sense to everyone involved. That said, many chose not to do so.