GTA Online adds a quality-of-life feature players have wanted for years then upsets everyone by paywalling it: 'One of the slimiest things they've done in a while'
If it was multiplayer only, there'd be no point in making it. All the stuff would already be going into GTA Online.
Red Dead Redemption 2 single player was amazing (and to date the only game that I would apply Ubisoft's laughable AAAA label to), despite them knowing that online would be an absolute milking machine.
I still have some faith in Rockstar. I don't expect any single player DLC for GTA 6, but I do expect the single player to be pretty damn good. They know what they're doing with this kind of content.
Based on how GTA V went I think it’s fair to say that the team working on monetization is far bigger than the team making the single player content lol