Why not have an optional mechanism where older cards from Wild / CL can be staked/put to work unchaining the Soul Bound cards for a set time?
The more BCX / CP staked per card the faster they unlock.
Even if implementation is after the 180-day DEC purchase window, it would be better than perma binding the cards for poorer players.
This way the Whales can pay DEC to unlock cards early and we mere mortals can also unlock our cards, but it will take longer.

Expanding on this - there could be a requirement for summoners to be staked (uses for older/non meta summoners) and certain splinters of monsters (Fire requires Fire, etc). Could even go as far as "sacrificing" (burning) an older reward card that was overprinted to speed up the "unlock ritual".

The more I think about it there's tons of ways this could have been implemented that would involve different gameplay assets/lore instead of just...costing money.