"But these interests don't overlap within a sufficiently large and diverse group of people, as much as financial self-interest does."
I honestly don't think this is true. Finances are a tool to get security and comfort in our society but it's still just a tool to achieve the real goal. I bet if we had viable UBI that gives people their basic needs, most people wouldn't worry about finances.
At the end of the day, most people are employed in part because they want to (or need to) make money.
Therefore, the greatest common denominator for an arbitrarily large and heterogenous group of employees at the company is the paycheck.
This isn't really disputable. Your argument doesn't really counter this fact, either. Sure, UBI might remove that common need that nearly every employee has, which could change the calculus entirely... But we don't have UBI, and the GP wasn't making an argument about some hypothetical world, they were making a point about the one we actually live in.
I honestly don't think this is true. Finances are a tool to get security and comfort in our society but it's still just a tool to achieve the real goal. I bet if we had viable UBI that gives people their basic needs, most people wouldn't worry about finances.