The idea eventually, I believe, was that Clemenza would have turned into a disastrous liability for the Corleone Family, either by rolling over and turning informer, or maybe simply by making some stupid careless mistakes (the flashback scenes of GF II foreshadow this by showing Clemenza's rock-headed personality at times).

Also, at the scene of Vito's funeral, the thought of Clemenza betraying Michael is brought up when Tom Hagen says, "I always thought it would have been Clemenza."

Michael doesn't disagree that Clemenza would be loyal until death, but that Tessio "was the smarter one" and cut a deal for himself first.

Clemenza also foreshadows possible disloyalty when he comes to Vito asking to break off into his own family (attempting to back-door Michael in the process). Vito shoots down that idea in a hurry, but still it's on the record that Clemenza was at least thinking about it.