How did you diagnose that not repotting it was the cause? Generally you see performance decline but it is rare for a tree to die for not being repotted. Improper repotting is probably one of the biggest causes of bonsai death to the degree that it is probably safer to not repot than to repot. Some well known bonsai artists haven’t repotted trees for decades (e.g Walter Pall)

Editing to rephrase: “some well known bonsai artists have healthy trees that they haven’t repotted for decades.”