That is a valid point, but keep in mind, that the 6M packs will be offered to active guilds. These are for the most part players, who would have purchased big chunks of the packs anyway and this way, they can pool their resources. And on the general sale release day, the shop is not overrun with those people but only us ordinary users, who purchase a few to a few hundred packs.
The queue will be much shorter that way in my opinion. Whales and big guilds will have their fair share and money is not infinite. I guess, there will be plenty for us on the first day of the general sale and we still get the promo cards for the other 6M if we purchase in the 24 hours of release.
Seems fair to me.