Japan isolated itself from the world for 250 years. Pretty much set it back doing so.
TKP wants "immigrants must condition in Japan". Hyvva pyvva (if my Finnish spelling is correct). What exactly do you mean? Japan has about a 2% population of foreigners, most being Korean, Chinese, and Brazilian, and the first two largely were born here yet don't have citizenship.
The country's population is declining. Birthrate is too low. Colleges are predicted to have a 100% acceptance rate by 2050, which means anyone can get in because schools won't be able to afford to turn them away. The number of old people is on the rise because Japan has one of the longest (if not THE longest) lifespans in the world. Who is going to do the work here if there are no more Japanese around?
Answer: nobody, because Japan has a terrible policy about immigrants
Perhaps you meant something else. Multiculturalism could mean no more influx of outside values. No more Mcdonalds (fat chance, pun intended). No more American or UK music or movies (unlikely). Fashions? Sorry, you can't avoid them in a world that itself is getting smaller.
Yes, it has been reported that more kimonos are bought outside Japan than inside.
Yes, fewer Japanese learn how to put them on.
Yes, more Japanese go to Hawaii for weddings than have them in Japan.
Yes, arranged marriages (omiai) are far fewer now than before.
But, culture is not static. It evolves. Japan does not have to sacrifice major cultural changes just because of globalization. In fact, globalization is what will probably keep Japan afloat!