What if?
Ok, let's put it this way. I play Fallout: New Vegas, that came out in 2009, four freaking years ago. It already looks better - graphically speaking - than either Saints Row 4 or GTA 5. And it's four years old. Skyrim came out in 2010 and it looks a LOT better at 3 years old. So if we assume the people buying GTA5 (I'm not, never got into the whole gang thing. I don't even like the gangs in Deus Ex 3 and all I have to do with them is kill them.) are in it for the graphics, then it will bomb, 100%.
If these people are in it for the music or the culture, then it'll do well, so no chance of failure.
But considering the previous success of the series and how awful the GTA4 graphics were, I'd say the crowd that likes this just likes the gameplay. So it'll sell well.
What if? Not gonna happen.
However, if somehow a temporal vortex opened in high orbit and sucked all of Compton into space? Well...I'd be selling my Rockstar stock, if I had any. Without its small, loyal audience, GTA as a series would be doomed. And I can't think of any audience for any of Rockstar's games that is composed of many other people, so the company as a whole would be hosed.
Rockstar would be wise to pay those gang voice actors very highly. They REALLY need them and their culture to stay alive.