It doesn't work like that.

There is a certain reward pool in Wild and next to that, there is the card supply with a certain combined value that can be used to earn part of this reward pool. Introducing a tax pretty much equals reducing the reward pool. What will happen is that card values equally will go down. This makes the card value drop from Modern to Wild even bigger and nobody in their right mind is ever going to want to pay for packs knowing this.

Low Old Card Prices = Nobody buys new cards/packs = No Money to Support SPS Price = Lower Earnings = Card Prices drop more = Dead Spiral/Negative Feedback Loop

High Card Prices = Willingness for new players to buy new cards/packs = More Money to Support SPS Price = Higher Earnings = Card Prices Increase More = Positive Feedback Loop