Michael knew with the way he let it seem that the family was just about done, with so little left that a smart person would be the one to try and cut a deal and get out with something rather then nothing. It was just a smart business kind of thing to do.

Clemenza would have rode the ship all the way down to the bottom. Loyal but not very smart, Vito and Michael both knew that it was a smart move to leave. In fact you have to some what blame them for the false setup that caused Tessio to be disloyal when he looked at what was happening.

By staying loyal as Pete was/did, they knew they could now trust him to go foward with them into the future. It was almost like checking the loyalty of your men in a way. To see who would be behind Michael once Vito passed.