If the number of cards is the real problem, why not use a "sacrifice" system ?
By burning (and giving up the DEC burn rewards) some cards, we accumulate points and we can use these points for unlocking soulbound cards.
Similar to lands PP, older cards can be burnt for more unlocking points.
The result is that mostly "weak" and unusable cards will be burnt, reducing the number of useless cards on the market and paradoxically giving them more collection value since only a few will remain available.
This is even better than using DEC to unlock since it will encourage players to buy and burn the cheapest cards, cleaning up the market and giving more value to the remaining cards.
I honestly think it could be a great occasion for the game to get rid of useless cards by burning them in exchange for unlocking cards.