That is indeed the main reason for the Soulbound card. I guess they want to go to a model where the main 'play2earn' feature is the ability for players to sell their cards on the market. Not sure that will be enough though. The setup right now is not ideal with the game having company revenue throwing some bones to SPS holders and a proposal system that is very selective. If the soulbound reward card idea gets put up as a proposal I would be surprised if it passed.