Considering how badly he struggled to hold it together during his horrible stint with the Wizards, I'd say it would be a disaster. Sure, he had some moments of brilliance, and even several very impressive games, but the majority of his performance was sub-par at best. When it gets to the point where it is no longer uncommon to miss a wide open dunk, to turn the ball over or to get blocked regularly, you're past your expiration date. Couple that with his nagging injuries from the last time around that heavily hampered his performance + another 8 years of aging, and you have a recipe for disaster.
There's already the argument that he tainted his legacy with the last comeback. All he's going to do if he tries to comeback again at 50 is go from a solidified legacy, to merely being remembered as the self-absorbed, embittered attention %!+%% that couldn't let go of the spotlight, even well after his physical abilities had diminished. As a huge MJ fan, and someone who will defend him as the greatest ever to play the game during his prime, I hope for his sake he doesn't do it (even though I can see his ego driving him to do it).