What a strange comment?! Perceived high achievers and high potential talents receive well above company average salaries or salary rises all the time in competitive industries like software, without waiting to see whether that person starts interviewing at other companies and what that offer is! They can't receive well above company average pay offers if the company's collective bargaining arrangement doesn't allow them to offer anything other than the standard union-agreed pay rate even the below average workers get. So they earn less.
If a company is willing to pay a person more than they're willing to pay their average and below average staff, but a union rule forces them to pay everyone with that job description the same, that person is obviously earning less money, not more money. I'm genuinely amazed a concept so simple can generate so many objections