I question just how true this really is.
People are starting to treat Dan Houser, Leslie Benzies and Lazlow the same way they treat Kojima, attributing absolutely everything to them when the fact is there are plenty of Rockstar "regulars" that, as far as I'm aware, have not left or distanced themselves from the company. (if you can provide sources proving otherwise though, I'd love to see them because I'm searching and can't find anything)
When it comes to writing, people forget that there was a writing team behind most if not all of Rockstar games dating back to Grand Theft Auto IV that consisted of Dan Houser,
Michael Unsworth and
Rupert Humphries. That isn't even getting into the other dozen or so people that helped with pedestrian dialogue and banter.
When it comes to producers, you have
Chris Choi who has been a producer since LA Noir and he recently became the VP of Development. David Kunkler has been a producer since Red Dead Redemption,
Neil McCaffrey since The Warriors, William Mills since Manhunt 2,
Josh Needleman since Table Tennis,
Rob Nelson since The Warriors...
There's plenty of people who have been at Rockstar for years now that are still there. Game development, especially games as large as Rockstar open world games, shouldn't really be attributed to a single or even handful of people.
Edit: As for adding online content to GTA V's single player, there were glitches to do it back in the day and there are still mods that can do it on PC without any adverse effects. I feel like this wouldn't effect consoles differently if it could be done, so I'm going to say best case scenario it's them trying to preserve the fact it is set in 2013, and on the worst case scenario it's probably just so you'll spend money to use them in Online. (although IIRC you can still pick new vehicles to use in races without buying them, correct?)