Getting rid of Gamestop is better described as a symptom of publishers' motivation, not the primary reason. I would argue that the primary reason isn't so much getting rid of retailer middlemen but procuring the data that comes with you having to be online to play your games.
Rest assured platform owners are collecting every tidbit of your usage data they can get their hands on - what you play, how long you play, what you search for, what sorts of DLCs you buy (and don't buy), what sorts of music you listen to, what movies you watch, what apps you use, etc.
Physical games on-disc mean you don't have to be online. It means they have no way of tracking you, watching you, and mining and selling that data to figure out what gets you to buy ... and what doesn't.
That data is worth every effort to move players to digital. It's certainly worth a WHOLE bunch more money than whatever they're allegedly losing to Gamestop and retailers.
Ultimately, this is all about having more control over their products, not just the game itself, but all of the information that flows in connection with it. Before, publishers had to rely on third-party surveys and NPD sales information, which had a host of accuracy problems, not to mention delays.
With digital, there is no lag, no missing information. The platform owner knows at any given second what it sold, along with a bevy of other usage information.
Too bad the average consumer has no idea what he/she is really giving up for the "convenience."