I've heard many different explanations for why Michael replaced Tom with Vito for consigliere.

The first one I've heard was Vito was dying and getting old and Michael wanted to use his wisdom as much as possible.
The second one I've heard was Tom wasn't a wartime consigliere, but his face of disappointment when that was announced almost convinced me he had the ability. Or did that mean he was vulnerable to getting attacked?
But the best one I've heard was how he was testing Toms loyalty, especially after the betrayals in the family.

What do you guys think? Tom is one of my favorite characters.