Twitter is a much better social network in Japanese than in English. You are more likely to encounter people who want to discuss hobbies than being trolls. I think they'll stick to the end, just like the Titanic.

Making the jump seems a little challenging right now. Some Japanese people have tried Truth Social, even dominating the trends with Japanese keywords. I'm not sure if they'll stick there given its context with American politics.

As Twitter became popular in Japan with the 2011 Earthquake, the migration to Threads might be another earthquake away (which happens often here).[1]

I wonder who will be the core users of Threads in Japan. Twitter was popular among nerdy "Otakus" who often engaged in anime, gaming, or electronics. Facebook/Instagram always tended to be avoided by that demographics. Instead, Instagram in Japan is popular among young women, who post selfies or food pics. However, I'm not sure if they'll be receptive to a text-focused service.

[1] Side-anecdote: Twitter is the best earthquake detector. Whenever you feel a shake, Twitter users will confirm it sooner than traditional news sources.