We have to keep in mind many cards will be locked on land. And we are not talking about level 1 cards, because the level of the summoner and monsters matter, as will the edition and probably if they are GF or not.
It may seem 15 mil is a lot in all, but I think it won't be. Plus they added those bonuses for high volume purchases, I expect a few million of them to go out in the first few days after the general sale is opened. Then, there's time for a year to sell the rest. And there are way more active players now than a year ago to consume them, even if nobody else joins (which is obviously not going to happen).