This is such a weird notion. Waves of immigrants have created some of the biggest most economically productive cities the world has ever seen. If Tokyo goes this route, only greatness lays in its future.

The rest is just nostalgia, and that's fine. But realistically, a big piece of Japanese culture will be well preserved in the new culture that will emerge. As it is, we've stopped lamenting the arrival of Buddhist influence on the island, so too will other immigration influences.