If he constantly comes back, wins, and then leaves, it's going to get old.
If he comes back and loses, it ruins the point of the character and his story.
If he stays away forever, he's different from everyone else.
There's no positive to bringing him back and he loses all that made him unique when he returned.
Well it depends how he is used and what motivation he would have for returning. As I said the only way he would come back is if he was forced out of retirement which would make sense. Remember even though Kingsley got away, he will always have to look over his shoulder for the rest of his life now that everyone knows he was the Hobgoblin. So many people would be gunning for Kingsley given the fact he got away such as Norman, Fisk, SHIELD, Interpol and other organizations-Plus, Betty Brant would also have tried to track him down along with Spiderman for killing Ned and so on. So even though Kingsley escaped all those years ago, he knows that he will never be safe wherever he went. All the money in the world won't protect him from all the enemies he's made and so I could see him becoming the Hobgoblin again to protect himself which would lead to various other stories.