Normal anime logic would be character A trains character B so they can realize their potential while not dying in the process. There is no good reason for Unohana to die. She doesn't even a single serious fight on-screen ffs
I mean, I really don't think there was time for a training arc, there. By the rules set in place by that particular universe, that was the most effective thing they could do. They decided "This is how X character powers up," and they ran with it, which is a typical anime trope. I get not wanting Unohana to die if you liked the character, but "logic" really has nothing to do with it.
Also, she technically was "training" him.
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Seplito Nash, Smelling Like the Vault since 1996 Step FOUR! Get Paid!