I believe that rather than just getting stronger by spending money, it should be made clear how far you can go without spending anything. Even if the same stats are attainable in-game, if the time it takes to reach them is excessively long, it's hard to see it as a real choice.

For example, if there are paid upgrades, how about showing the acquisition rate and time required for the free path in advance, and disclosing how wide that gap becomes each season? While I understand the need to cover operating costs, the competitive rules of play need to be understandable for everyone if you want them to stick around for the long run.