Responses
It's the same that happened to Souma's dad
Well, Isshiki did not get praise for his actions it was rather expected for him and his family decided to burden him with the role of succession that he got tired of cooking.
Kinokuni did comment about that right? The part where he got disowned?
I think that Isshiki-senpai was really motivated by the effort and time that Kinokuni put into trying to master what he has done in a genius short amount of time.
It's only so long before a genius gets tired of the praise and what he can do and then gets bored and demotivated to do anything worthwhile.
He decided that he would probably advance the "traditional" cooking of his family line by making unconventional twists to it and probably got disowned for doing such things. But by doing such things he managed to use his talent to put wild game into a delicate dish.
Now that's talent used right :D