Tsunade was getting very old and tired, being involved in 2 (or 3) Wars.
Similar to 3rd Hokage.
Note all 4 Hokages, before Tsunade, die fighting for the Village.
As for Kakashi to be the next Hokage, there was a hint, when he was about to be 6th Hokage.
It will be disrespectful not to appoint him, as he was nearly being appointed. (Kishi like to give hint)
There is one very important hint
Remember what was Sasuke 'revolution' - to kill all the Kages. (another one of Kishi hint)
Therefore, there could be some argument, but at least Kages did respect Kakashi word,
Refer to
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Thanks for this thread...