Shawn got to go out on his own terms.. In the mainevent at a wrestlemania and he got to walk off into the sunset as the greatest in ring performer in WWE history. I think it's probably the grandest exit we've even seen in wrestling. It would have been 2nd had Ric Flair actually walked away after WM24. The only way I see Shawn having another match or run would be if it was huge. I would hate to see him retire after main eventing a classic match at mania only to have him return 3 years later to face Alberto Del Rio at Night of Champions.. But if he did have one more huge main event wrestlemania match against.. HHH or Rock ect then I can accept it.
People get so upset that Flair worked for TNA... he wasnt a full time wrester, in fact he hardly wrestled at all. No one was upset when he returned to WWE as Batista's Enforcer or when he and Jericho were a big part of the build up to WrestleMania 25.
As for HBK, he didnt retire because he was too old to continue full time (actually part time, HBK hasnt wrestled a full time schedule since 1997) he has chronic back issues, related to Royal Rumble 98. That was why he didnt wrestle at all from 98-Summer 02 except for losing the title to Austin at WM in 98.
HBK came back for the same reason Flair came back in 02. To put a positive cap on their careers and legacy. For Flair it was a chance to end on a positive note after Bischoff & Hogan nearly S$#%canned his career in the late 90s. For HBK it was a chance to give back, to make up for the guy who was by many accounts including his own a total selfish jerk during his heyday, refusing to lose, refusing to elevate anyone, going out of his way with his "Backstage politics" to protect his image at all costs, even at the expense of the company as a whole. No matter what Hulk Hogan does or how much one can celebrate his past accomplishments he will never get away from that same stigma. HBK didnt want to be remembered that way. He came back, he turned in good perofrmances no matter what the match result, he elevated Jericho, Orton, Chris Masters (he didt take advantage of it), yes he won his share of matches but he did what was asked of him and helped build the product. Same with Flair at that time.
Should HBK come back ? He probably cant come back for more than a match or two due to health concerns. You get the impression that HBK is happy at home with his family, away from the bizz, so why give that up ? At least he found some peace, I wish Brett Hart would do the same.