The emperor was forbidden to perform any political activities by constitution. Simply stating his wish to abdicate is considered a political activities, so the emperor cannot even say that.

This time, he implies he wish to abdicate by saying "When the emperor dies, Japanese economy will face a serious shock, which I want to prevent it", which implies in the time of his death(I don't know the polite way of saying die in English, in Japanese, we have a special polite word for the emperor's death), he dies not as the emperor so the emperor's death doesn't happen(This is just observation. the emperor can't directly say that obviously because the constitution prohibit his political activities.)

It is us, the Japanese, who can change the constitution. To change the Japanese constitution, it requires the national vote.

Also, it is my understanding that the emperor is not, by definition from the constitution, a Japanese person.

The Constitution defines:

The emperor has no political power, his basic human rights is restricted, has no freedom of job choice etc.

Japanese people has political power, basic human rights, freedom of job choices, freedom from being enslaved etc.