Moe was small potatoes. Had Barzini's attempt to kill Michael and become the big cheese been successful, he would have never kept his end of the deal with Moe Green.
It's not even clear that Moe actually had a deal with Barzini. "I talk to Barzini. I can make a deal with him and still hold onto my hotel..." Moe might have been bluffing Michael at that point. Or he might have inferred that he could make a deal with Barzini, without actually having broached the specifics with him.
