Would you care to speculate how other shihan of Tada's generation would have answered your question? Eg Yamaguchi, Arikawa, Tamura, etc. The late Yamaguchi in particular still has his followers teaching at Hombu Dojo (Endo, Yasuno, Kuribayashi) and also there is Takeda Yoshinobu. Would Yamaguchi have said that his aikido would die with him? Did he have a different model for transmission and emulation?
I do not think that Yamaguchi Sensei consciously designated anybody as his followers and I am certain that his aikido has already died (and Arikawa Sensei's), in the same sense that Tada Sensei's will die when he does. Hiroshi Isoyama in Iwama is another example.
Tamura Shihan is somewhat different. He went to live in France and has never returned to live in Japan. He leads a huge federation in France and has produced a large number of yudansha, as has Christian Tissier. But I do not think there is any one heir to 'Tamura Aikido'.
I will have more to say about this in future columns.