I like how you basically gave a good reason for the existence of digital codes by the time you were done.
Buying a code for a digital game from Gamestop is something I would never do.
Actually, many years ago I did buy a digital code for Teenage Mutant Ninja Turtles: The Arcade Game from Gamestop. The game was de-listed on the Xbox 360 store, but Gamestop still had some digital codes.
Couple of things:
1) This is all retailers, not just GameStop.
2) PSN STILL has no gifting system (unlike the competition) so this remains the only way to gift digital games.
3) A number of people still don't trust Sony with their card info, and don't like buying more credit than they need, or end up buying too little because of tax.
4) Places like Best Buy don't let you spend GCs on PSN cards, but do let you spend them on digital games.
5) Ultimately, this gives Sony a stronger monopoly meaning worse discounts.
This might not be the death of PSN Humble Bundles (for charity) since they're limited and I'm 99% sure Sony needed to specifically sign off on each of those anyway. I can see them treating those as special cases as opposed to constantly losing "potential revenue" to retailers.