It depends on how it was worked. I like my Goldberg idea, though it's entirely fantasy booking (but the mere thread title encouraged that.) In another situation, the person who defeated Undertaker wouldn't have to remain undefeated for just as long or longer than Taker. It could be someone who had two or three wins before defeating Taker (just as a rough starting point), and then afterwards, they go on to have maybe five or six more wins after the big victory, which puts them around 10-0 or something when someone else finally defeats them. Hell, they could have as many wins as someone like Edge had (8-0) when he lost his streak, but the fact that one of those wins was ending Undertaker's streak would maximize their streak's legitimacy big time.
This could work. It wouldn't have to be someone who kept winning match after match after match for the next twenty years or something.