I think inevitably he was bound to end up at light heavy. I don't think he'd be able to keep his weight down for very long at middle, beyond perhaps two years and maybe a dozen fights. So yes, he would have moved up and still accomplished great things.
He would have been forced to move up before too long, but the mere fact of him having fought at middleweight would have enhanced his p4p standing in peoples perceptions. Tommy Burns is another fighter who could have underlined his p4p greatness, with a couple of key fights at middleweight.
Michael Spinks would not have wanted to run into Marvin Hagler at middleweight. Marvin at middleweight when he was active was strong and consistent. Although Spinks would have beaten Marvin at 175, and Marvin should have taken the chance. Had Thomas Hearns beaten Marvin in 1985, he was going to move up and fight Spinks the next year.
Tommy was very confident after stopping Duran. I think he really thought he could stop Hagler and then go up and beat Spinks. Sometimes confidence or overconfidence makes a guy think he can beat anyone. Spinks would have been tough for Hearns, especially had Tommy moved up without getting comfortable at the weight.
