Again, while the Pentangeli storyline would not fit Clemenza, I don't see why people think he would never have turned on Michael.

The overriding theme of Michael's donship was his driving away of those closest to him, whether well-versed in Omerta or not. Tessio, Altobello, Kay, etc. Why would Clemenza be immune to this?

While I, too, like Clemenza and would have been ripped apart to see him betray Michael, that would have been an even more compelling storyline than a previously unknown character do so.