I think the focus right now is on a lot of how the match would go from a wrestling standpoint, which I kind of feel, shouldn't really matter when you apply it to these two. Hogan/Andre, Hogan/Warrior, Rock/Hogan even Rock/Cena (both times), none of those were what you would call classics wrestling-wise, but the moment itself, the crowds reaction, and just the mere image of the two standing in the ring face to face is enough to get people to turn heads.
One could make an argument about them being the people in those matches being over with the crowd, and you'd be right, around those times the people were hot, or the people around the product knew who these guys were. That might be somewhat of an issue around this time, because it's questionable how many people know who Sting is, everyone knows who Undertaker is, but Sting hasn't really been around the public eye (not even in TNA, because TNA really doesn't know how to promote its stars, case in point RVD, that's not a bash to TNA, but it's something they need to work on), so it's questionable how the reaction would go for him. Around the time of Warrior/Rock/Hogan these people were still fresh in peoples minds, and while it's no doubt people will know who Rock/Hogan is, it's debateable on the amount of people that would know Sting or remember him on the product of today, might or might not be an issue.
I'm not saying though, Sting/Undertaker can't put on a good match, in my mind I think they could pull off a great match, but the match won't be remembered for the wrestling, it'll be remembered purely for the moment, and the fact that these two men (greats) stepped into the ring together. Just showing a clip of Sting/Undertaker after the match (if it ever does happen, which I doubt it will and have for a while), it would be remembered as a classic, if not for that image alone, and in that end, that's really all that matters to WWE, it's history, it'll become apart of history and it'll be remembered.
I don't see a ***** match coming out of Undertaker/Sting, but in these kinds of matches, you're honestly just about never going to.