Only correct answer is convenience. DD will never fully take over because of the sizes of these games. Most ppl simply don't have the connection to justify downloading numerous games at 50gb apiece. For myself, I'd rather do it that way, as I'm partial to the ability of not getting my lazy ass up to switch games when I wanna play something different, but that's just me.
I also agree 100% with you point on price. They have/had most Next-Gen games on sale for $49 at different stores, yet the digital version of those same games were still $60. Even on Amazon, and their newly opened PS4 Digital Store, the digital versions of games were still the same, while the physical's were cheaper. It makes absolutely no sense. When that store was first announced, this was my main hope, that they would have, while maybe not on the level of Steam-like sales, atleast sales that would be in line with what they do with physical discs. If they're not gonna do that, then there's really no point to the store, other than not having to go thru PSN to buy. And if they bring Paypal to PS4 (which has to be inevitable, as it's now on PS3), then there's literally no point to it.