It would of been diasterous, because 1.) Orton got injuried the very next night on RAW. His legendary push, would of been all for nothing.
2.) Orton's character has developed in a good way over the years. Instead we would of had a very stale face Orton, who yes was over, but would of been very bland. He didn't have a great catch phrase at the time, so he wouldn't of caught on that well.
3.) We talk about streaks, and history of wrestling, and nothing is more sacred, then the Undertaker's Wrestlemania streak. One of the few guarantees in the this business. A guarantee we much enjoy. You hear how people retire, and then come back a few months later, making a career vs. career match not even worth a damn. Other stipulation matches that change months after the result. But, it's a sacred event, The Undertaker's streak. A meaningful entity that the industry sure stay true too.