As an old school wrestler, i can't see the Undertaker EVER approving of Cena ending the streak. I'm sorry, but someone as respected as the Undertaker must be slightly pissed off at the companies direction since 2000. I know he's a company guy, but he has a lot of sway and i'm sure he has pretty much full say on his character at this point in his career.
There's tons that have become disillusioned with it; Stone Cold, Rock, Jericho (has seen his interest wain over the past several years, only returning when he has nothing planned with Fozzy), Goldberg, Batista.
It should never end. There's nobody there that deserves it and it will be hard to find anyone that will. The only person that could ever end will be the person that takes WWE to the next generation. Creative would have to be 100% sure that they have found themselves the next Austin/Hogan, because looking at the company right now, it definitely isn't Cena.