Since Undertaker never faced Stone Cold or The Rock at WrestleMania...?

No. Simply put, the Undertaker is unbeatable at WM. It's just part of the legend and the story behind him. WWE made the greatest superstar in Taker and he cannot and will not be beaten at WM. They talked about ending the streak but Vince thought it was better for him to go 20-0 rather than bump a star who won't be there and doesn't deserve to break it.