He cleaned out WW for a while, but it had become a much better division by 2013. The overwhelmingly majority of people thought that Hendricks beat him, so a rematch was warranted. Lawler and Maia had become credible contenders, as had his teammate Rory. Those are 4 top guys that GSP needed to beat convincingly to truly clean out the division. Obviously, he decided not to fight Rory because they were teammates, but that also means deciding not to attempt to clean out the division. Then, within a year or two, Woodley and Wonderboy emerged as contenders. There was no shortage of fights for GSP.
The real shame will likely end up being that GSP never fought Usman. When Usman completely dominated Woodley to win the belt and move to 10-0 in the UFC at welterweight, GSP was 37 years old. As you said, GSP, at 36, looked great against Bisping. He should have come back to fight Usman. Now Usman has a chance to pass him as welterweight GOAT. Usman is only a few fights away from running his record in the UFC welterweight division to 15-0 with no split decisions. GSP went 19-2, but 2 of those victories were split decisions.