I think ultimatley it will be up to Taker, as it should be after the service he has given the company.
What i would say is that it could make a "Star" if they had someone on the cusp of super-stardom like Austin was when he fought Bret, then it can be used really well with Taker's blessing of course.
However its not worth ending it for the sake of ending it. Its a great honour and has become a integral part of Wrestlemania these days which is great coz it gives another big match an edge.
But like i say if Taker wants to go out with a defeat then they will but i really hope if they do that they give the rub to a younger guy with promise. However if i was a betting man i'd say Taker has 3 Mania's left and he'll go out undefeated.