Sure, he could be as great as Hagler. You've got to remember that Hagler didn't have a lot of great scalps until right at the end of his career. Then he fought all the guys people remember him for in his last couple of fights. It took a long time for him to become a draw and make it worth other star's trouble to fight him. Duran, Hearns, and Leonard had been avoiding him forever. He'd been trying to get Leonard into the ring for four or five years, just like Mayweather/Pacquiao.
He certainly can. If he unifies MW and beats a few top comers from 154 like Lara, Andrade, he will be as great as Hagler.
Well, Hagler turned pro at 19, Golovkin at 24. That said, GGG has to go on a SICK run the next couple of years to be in the same league historically as Hagler. I'm not sure who is out there to elevate GGG the way Hearns and Duran elevated Hagler. Canelo would be a great feather in his cap, for sure.
its probably too late on reflection - given 3Gs style will take its toll on his body soon. He should be wrapping his career at the top by age 36. but again its not fault he started late.