> How do you know the salaries wouldn’t have increased just due to baseball popularity and demand for good players?

For starters, baseball is not nearly as popular today as it has been in the past, but salaries have not decreased. But more fundamentally, we do have a demonstrative example of great outcomes for players with the union, whereas we don't have a counterfactual of what would play out. Since you're the one arguing that a different choice would have been better (or the same), it stands to reason that you should have to provide evidence, because in the absence of either side providing a compelling argument, the only data we have is "things sure worked out well for the players after they had a union".