It paid off, in my opinion.
Before R* announced it, I thought it would be ridiculous.
When they announced it and explained how it worked, I thought it could be interesting, but still wasn't too sure about it.
Now, after owning the game I think it would be hard, but not impossible, to go back to just one character. Each character brings a different feel to the world, even while freeroaming, and for me it extends the life of this game.
If R* does continue with multiple protagonists, I would like them to still do something unique with it. maybe have two protagonists that never cross paths until the final mission, and then you get to choose who dies or who gets money, or something like that.