I always thought that Sonny would have wasted the cream of the Corleone fighting force in relatively pointless tactical battles. The book suggests that hearing Sonny rant about fighting to the last man made Clemenza and Tessio uneasy. Certainly the people who actually were on the front lines would have even more reason to feel that way.
Sonny was a berserker who was most effective when pointed at an enemy and released, not when thinking. So I think that's what the Don was admitting.
I think there would have been a few defections during the war, a few more after the peace and perhaps more once it looked like Barzini was ascendant.
"When the snows fall and the white winds blow, the lone wolf dies but the pack survives."
Winter is Coming
Now this is the Law of the Jungle—as old and as true as the sky; And the wolf that shall keep it may prosper, but the wolf that shall break it must die.
As the creeper that girdles the tree-trunk, the Law runneth forward and back; For the strength of the Pack is the Wolf, and the strength of the Wolf is the Pack.
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