You don’t understand how people behave in traditional Japanese society. Not accepting a son's work is literally the highest form of praise. "Last work" is a reference to master swordsmiths, whose "last blades" were so numerous they could be measured in tons.
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I’ve been studying Japanese society and culture for 17 years. He was not simply “not accepting his son’s work”. He was calling it a disgrace. And Hayao is not a swordsmith, he is a salty grandpa that finds it hard to let go.