In still other words, although a blue square GTA might be kind of fun, it wouldn't be clear that it's "holy crap, once you get the graphics in this is going to sell at least 500,000 copies" fun.
Even if you'd given me GTA, the original 2D one, and said, "Hey, want to fund a 3D version of this?" I'd probably say something like, "Well, we'll fund you for six months: get the carjacking and open city tech up-and-running, and if you hit that, we'll fund you to the end of the project."
And in so doing, possibly miss out on the greatest franchise ever. I have no idea how long it took for Rockstar to get that stuff going. It may have been more than six months.