Minty, you are making a strong case that the income disparity was always there. I wonder if I could trouble you one more time to go back to your source one more time to check the attendance figures for the Red Sox and Cardinals during the time that the Braves and Browns were doing so poorly. This would help clarify how much of the poor attendance was due to lack of success and how much due to the market.
Typically, when a league expands, one of the major factors they consider in granting franchises is the potential for success in the different locations. If one location has far greater potential than another it is likely that that location will get the new franchise. Any new location that you might relocate the Expos to is likely to have been considered on the list of locations for potential expansion. Since that location did not get the franchise while Florida et al did, it is not likely that that location has significantly better potential than Florida. And since Florida did not work out, it is quite possible that the new location will not work out either.
Of course, it is always possible that it will. Other factors besides potential revenue do enter into the consideration, and the franchise committee might have simply judged wrong. But getting a new franchise (or venture of any sort) going is always a risky undertaking. And at a time when baseball has failing franchises, it does not make sense to scrape the bottom of the barrel to try to come up with new ones.