People are just acting stupid in here. 2 Fast 2 Furious made only 20 million more than the first one worldwide, but its budget was pretty much double the first one. It was still profitable, but its increase in box office was nothing.
People thought the franchise was dead when Walker left as well and they did Tokyo Drift, which made little money. When the original stars (both of them) returned however, it jump boosted the franchise back to life instantly, and from that point they've been building franchise into a different beast entirely, but make no mistake, Vin and Walker are the stars that made the franchise, not just one of them, and not the inclusion of The Rock.