My problem with this proposal is that i would hurt the ranked rewards.
After a player acquired a maxed soulbound card, any more copies of that card wouldnt have any usage without paying to manifest that card.
This especially would hurt players in high elo and those who buy a lot of energy, since they will be the first ones to have maxed reward cards.
Furthermore i think it would lead to a lot of frustration, since at some point (champ) player would earn reward cards, which would have zero usage without paying additional DEC.

What i would suggest in addition to the proposal, would be to make maxed soulbound cards usable on land without having to manifest them.

This would give player an incentive to earn more reward cards (after already owning a maxed copy), while also preventing players with multiple accounts from gaining an advantage.

It wouldnt be easily exploitable by one player with many different accounts, since each account would need to play in high elo to earn enough bcx to combine them in a maxed card.
In addition to that with the introduction of sps staking requirements botting with many different accounts will take another hit, particularly bots that would play in champ, since they would need a high amount of sps to earn enough chests to obtain maxed soulbound cards.

Any non maxed soulbound card would still be needed to be manifested to be usable on land.