I don’t think the show is rushing to choose a permanent replacement for Alex. In addition to Aaron Rodgers, several additional guest hosts have been announced – Bill Whitaker, Savannah Guthrie, Sanjay Gupta, Anderson Cooper and Mayim Bialik – and supposedly more are planned (I’ve seen rumors about LeVar Burton). Not everyone who is guest hosting is auditioning to take over the show, though.
I thought Ken J, Mike R and Katie C were all good in different ways. Ken really grew on me; I found his voice a little whiny at first, but I got used to it (or maybe the production team started using electronic means to beef it up a bit), and I thought he did a good job of being smooth without being slick.