I think it makes sense to require teams to
win with the players that got them to where they are rather than with a whole slew of players that the average fan never heard of.
Using round numbers that say a major league ball player gets about $100K per month, then the cost of adding a player for the season would be $600K, add to that 2 months @ $100K for the two players brought up for September, and the total cost for the changes is $800K. Of course, if the cost for that 26th player was greater than $100K per month, the total cost would be greater than $800K. If a team would increase their roster by 10 for September, that would cost $1,000,000.
The benefit for the union is that they get another one of their members on the clock,
plus they keep 8 or more minor league prospects who are looking for a major league job out of sight and out of mind until next spring.