I take a different view:
As I've posted before: I believe Geary was part of the plot to set Michael up for five counts of perjury. As a member of the committee he had to know that they were holding Pentangeli in reserve. He asked Cici if he ever got a direct order from Michael or if there was always a buffer. When Cici said no, he never got a direct order, Michael relaxed: he thought Cici was their top witness against him, and since Cici said he never got a direct order, the way was clear for Michael to lie under oath. Then the hammer fell on him. IMO, Geary deliberately set him up for perjury by asking Cici that "buffer" question.

I also believe Geary was being two-faced in his statements--the all-American political two-face. When he said "...these hearings on the Mafia are not a reflection on the great Italian-American people..." who did he mean by "Mafia" if not Michael? And when he said, "...it'd be a shame for a few bad apples to spoil the whole bunch," who did he mean if not Michael?

So, Art, your question is a good one. I, too, wonder why Michael wouldn't have whacked Geary--Michael was far too smart not to have figured out that Geary helped set him up. Why would Geary risk it? Revenge for that brothel setup. As we saw elsewhere in the Trilogy, seekers after revenge seldom think through the consequences; and anyway, the only thinking Geary did was with his dick.