Ex-Rockstar Game dev Joe Rubino confirms what we had suspected all along: GTA V's anticipated campaign DLC had been scrapped in favor of GTA Online content.

Grand Theft Auto Online has been so successful that it has led to the cancellation of traditional paid singleplayer content. Given how much money GTA Online has made, and still makes, this may not come at a total surprise. The live service mode has helped the GTA franchise generate a whopping $9.1 billion since Grand Theft Auto V's release in 2013, and is a substantial driver for GTA's $150 million to $200 million in quarterly revenues.
But putting together something as massive as GTA Online isn't easy. It takes a lot of work, and a lot of investment. Because of its incredible earnings potential, GTA Online has been prioritized over the traditional buy-to-play story content that used to be the singular expansion point for mainline GTA titles. In the past, this offline story DLC did not sell nearly as well as the main games.

The business decisions remain clear on a numbers level--just take a look at how much money GTA still makes, primarily because of GTA Online--but now we have new info directly from a former Rockstar Games dev.
According to former Rockstar cinematic artist Joe Rubino, the studio had been working on a standalone DLC starring Trevor, but the project had been "shelved" because of GTA Online.
Rubino explained in a recent interview with Spanish Grand Theft Auto-focused YouTuber SanInPlay:
"A lot of that stuff went on to go into GTA Online later down the line, I think. It's not like they wasted it--it was really, really good."
"But you know what happened? When GTA Online came out, it was so much of a cash cow, and people were loving it so much, that it was hard to make an argument that a standalone DLC would outcompete that.
"I think looking back now, I would say that you could probably do both. But that was a business decision that they made and you know, I was a little upset about that. And actually that was a lot of reason for me being a little sour at that time because I was like, this is awesome, let's finish this--and they shelved it."