This also allows me to give keys to other vendors, like Humble or GreenManGaming, who do take a cut (in some cases close to Steam's 30%, in other cases closer to 10-12%). Those vendors may choose to discount my game beyond the discount I offer by giving up more of their margin to the consumer.
It is not uncommon to see games on Steam where the origin of user reviews is 50% Steam purchases, 50% off-site key activations, which implies that Steam's effective margin may be as low as 15%.
Now, I think most independent developers are willing to sell on any platform who will have them and for whom the marginal costs of submitting / uploading are lower than gross sales, so certainly I don't think Epic is making a bad move here and I suspect they'l get decent developer uptake, but I also think the sort of "topline sticker comparison" obscures some of what's going on.
To some extent there may be value in having this discussion a week from now when Epic has rolled out the store and we have a better idea of how other considerations stack up.