You want to incentivize people to play the game day in and day out. There should be things that cannot be purchased and must unlock with play. The cards being played have value to the player in gameplay, even if they cannot be traded or sold. Also, even soulbound NFTs can be sold. I mean, you do have to sell the entire account, but it can be done.
RE: Is it not a concern that the introduction of "soulbound" cards feels like another step in restructuring toward becoming just another card game that may no longer need or want a P2E aspect or even utilize blockchain technology in general?