It's all about financial incentives and this far down the pipeline it ain't happening. GTA Online is a multibillion dollar business, GTA V Singleplayer not so much.
Games that followed these incentives to the end of the line joined the hegemonial blob though, becoming indistinct and uninteresting in the long run.
Jack of all Trades, Master of None, but people will buy masters only in the longrun.
And take for example minecraft, which proofed that the whole market is willing to dissappear for years on end to into a simulation were the player can tell its own singleplayer story, uninterested in any content any company has to sell.
One more longterm succesfull open-source sim of that kind without online shop and half the industry is forever gone.
I think you're overestimating what's necessary for online to be successful. GTA V is almost 10 years old and still raking in cash from online. The focus does not need to be on online play for it to be successful, they have billions of dollars in evidence that confirms that.
I would say that GTAV is focused on online play. A lot of the content is only accessible that way, ESPECIALLY the content added after launch. If you look through the list of new missions/locations/vehicles added to the game[1], they're almost exclusively only in GTAO
I have to admit that I got GTA V for free. But still even if I had paid the full price I would have been entirely happy with the single player story line.
I will likely buy next GTA on launch if it launches at same time on PC as consoles.
