Ever since day one of his arrival in TNA, Sting's been nothing but gold. He's been their center piece, and the guy has been rewarded every step of the way.
TNA gave Sting a total of FIVE world title reigns, one of which is the most prestigious World title EVER in the NWA World title. TNA also gave us matches like Sting vs. Kurt Angle, revived his career, and even added an extra level of depth to his legacy. The whole "Sting not being in WWE" is all because of TNA, and it's helped raise the awareness for him than anybody could possibly imagine.
To sit there and say that jobbing out to the Undertaker ONE time would out shadow everything he's done in the last 10 and a half years is not only asinine, but ludicrous as well.
The OP has a valid point. If Sting goes to the WWE, it shouldn't be to face The Undertaker. If anything, he should be given the same level of respect that Ric Flair was given. Sting's not as old as you may think, and with a WWE medical staff, I'm sure he can become even better. Sting should be utilized to help build younger stars like Ryback or even Daniel Bryan, as well as to revive legitimacy to some titles. Hell, giving him the US title and letting him run with it would do wonders.
Facing Undertaker has ONE outcome, and only one. You face him, people talk for a few months, and then you're forgotten. Sting, having the successful career he's had, should not be relegated down to that within even the slightest bit.
Flair was added to the streak a year after WCW fell. So, if Sting deserves what Flair got, then by all means he should be added as well.
Sting in TNA is the whole tree falls in the woods conundrum. If no one saw it, did it make a sound? A majority of wrestling fans have no idea who Sting is and if they do, haven't had any idea what he's been up to since the match against Flair on the last Nitro.
The "NWA title" that TNA branched from isn't prestigious at all. The NWA title was diluted the first time when WWWF broke from it. It was diluted further when the AWA broke from it. It was diluted further when WCW separated it's World Heavyweight Champion from the "International World Title". Then it was made historically irrelevant when Shane Douglass won a tournament for it, vacated it saying
"In the tradition of Lou Thesz, in the tradition of Jack Brisco of the Brisco Brothers, of Dory Funk Jr., of Terry Funk-- the man who will never die. As the real Nature Boy Buddy Rogers, upstairs tonight. From the Harley Races, to the Barry Windhams, to the...(Douglas grimaces in a thinly veiled look of disgust) Ric Flairs, I accept this heavyweight title. ... Wait a second. Of Kerry Von Erich. Of the fat man himself, Dusty Rhodes. This is it tonight, Dad. God, that's beautiful. And Rick Steamboat, and they can all kiss my ass! (throws down the NWA World Heavyweight Championship) Because! I am not the man who accepts a torch to be handed down to me from an organization that died-- RIP-- seven years ago. The Franchise, Shane Douglas, is the man who ignites the new flame of the sport of professional wrestling! [taking the ECW title] Tonight, before God and my father is witness, I declare myself, The Franchise, as the new ECW Heavyweight Champion of the world! We have set out to change the face of professional wrestling. So tonight, let the new era begin: the era of the sport of professional wrestling, the era of The Franchise, the era of the ECW."
Between then and TNA attempting to use it, it was held by the likes of Dan Severn and mike rapada, so please don't act like the NWA championship that TNA used was the same one that Flair and Steamboat had wars over, or was the same championship that Thesz held. One was for the best in the industry at any given time, the other was for a bunch of jobbers who couldn't make it big anywhere that mattered.