He is American television's best intellectual interviewer in the tradition of David Frost, Dick Cavett & William F Buckley & he has no problem talking with high-level intellectuals.
Not to mention he has a bipartisan reputation & can interview both sides of the American political divide.
Here are my choices for the replacement:
Terry Gross from Fresh Air. The best candidate around.
Melvynn Bragg from In Our Time
Brian Lehrer from his show (probably the most bipartisan interviewer that I know)
Leonard Lopate from his show (Brian Lehrer's colleague. His shows are more intellectual than Brian's, I think)
James Lipton (although he is mostly interesting in the art of acting).
Others:
Barbara Walter
Larry King
Oprah Winfrey
Matt Lauer
Anderson Cooper
Fareed Zakaria
(probably not, they are too big already & have their plush jobs).