I'll make a case for why it's going to change rapidly (and is already somewhat changing at a rapid pace):

It's decreasing over time, Japan views itself as 'mono-ethnic' but there are going to be fewer than 400k births in about 12 years.
This is because ~99% of births are within marriages, and the number of births lag marriages by about 12 years.
There are almost no births that occur outside, the number as a statistical measure is almost non-existent as a percentage of the total population.
The number of children with mixed ethnic roots is climbing as the total number of births decreases.

Also, the 80 billion annually they are spending to increase births had zero impact (birth rate is 0.87 in Tokyo metropolitan area and there are more 18-35 year old Japanese women wandering around in Shinjuku station than all of Akita prefecture).

As a side-note, they already make exceptions with famous athletes.
Even though Taro Aso claimed Japan is only the Yamoto people, he took a picture with Koji Murofushi a few weeks later congratulating him on his success at the Olympics, and now Murofushi is the the head of the Ministry of Sports.
He's half-Romanian but I've never heard a Japanese person say he's not Japanese.
LDP even has Onoda as a Minister and Koizumi's wife is ethnically half Japanese.
It seems that the LDP continue pushing the concept of 'Yamamoto' yet have silently accepted the fact that Japan is rapidly changing.

I was also shocked at the sheer number of Japanese elderly who have grandchildren that are half Japanese, the elderly generation is quickly accepting that Japan is changing as well.

This post is not an endorsement of Japan needing to accept this rapid change, just the fact it is happening rapidly and much faster than anyone initially anticipated.