2 reasons:

1. The distribution of rewards is going to change to better reward higher leagues. Right now you get a better ROI having a lot of low league accounts compared to fewer high league accounts. This extra incentive will increase the value of leveling up cards, increasing demand.

2. "Ghost" cards will no longer contribute to your earnings. The majority of accounts out there utilize only ghost cards and don't buy or rent copies of the cards they are playing (in large part because there is little incentive to level up, as stated above). Needing an actual copy of these cards should significantly increase demand for them.

It will also provide a cap on how many people can play, which will derail inflation IMO. Right now there is virtually no limit to how many people can earn from the rewards pool. Remember, the reward pool doesn't increase just because more people are playing, so more players = fewer rewards for everyone. Especially since they don't need copies of the cards they are playing. Once they need copies, more players will still mean a smaller share of the rewards, but it will also mean more players fighting for a limited supply of cards. Lately we've seen a booming number of accounts but no increase in card value as a result. I think we are in for a major correction there once this hits. Either the accounts drop out leaving more rewards for everyone with cards, or those accounts go buy cards and drive up price.