I don’t know that the goal of making short stories mainstream needs to be the same as making unknown authors mainstream. They are two different animals. The reality as far as I can see it is that the world is shifting more toward branded entertainment in general. A stand alone short story from an unknown author is a tough sell. Branding GJS (or any other collection) gives it a place in the market.
A good comparison might be to the exploding podcast market. While nearly anyone could put out a good podcast today, websites like Vox and people like Malcolm Gladwell have opted to join the Panoply label. This gives them a brand association and a foothold in the market. It also gives Panoply a brand ID. I might list to Malcolm Gladwell, because I like other Panoply stuff.
The same could be true of fiction. GJS is looking for elite writers, that’s their brand. That leaves plenty of room for other short fiction brands in the marketplace.