Well then just like 99% of the people still want the groups to take their coin when it drops - and they always do, I would argue that they’re agreeing to sell the coin regardless of oonah either way since they don’t want to take the risk.
"They always do" - true, but a mechila
at the time of the transaction is required by the seller.
Likewise, when it goes up, a mechila at that point in time is required by the buyer.
The large number of people that wish to back seems to indicate that they are not interested in being mochel, which they are halachically entitled not to do.
Incidentally, on a practical level, the coins will never go down by more then 1/6th as they are returnable to the mint.