The problem is that if Japan ends up with a population of 64 million in 2100, that is nothing like the situation 100 years ago. Almost all of those 64 million people will be retired and not creating any economic output, but requiring a lot of care. It's questionable whether the small working age population could even provide that care, much less all of the other things that a functioning society needs.

Now it's certainly possible that AI will eliminate the need for anyone to work. That is a different question though than conventional demographic prediction.