Theory: Online
Theory
I think the reason that Rockstar is highlighting that gta6 is a "single player experience", is because they are going to follow the route of the other major players in the industry:
A free to play online mode, with paid upgrades and microtransactions. I am thinking it will be like Battlefield, and COD basically: you can play but with limitations. They made something like 5B in microtransactions so I think the story mode is basically covering the expenses and then online mode will be pure profit and with 39 million copies pre-ordered they already have a great start for the f2p online mode.
Just my two cents.