immigration didn't save UK and Germany, and it will not save Japan either. no one in their sane mind will look at Western Europe and say 'yep, we want the same thing here. we want to be minorities in our own cities, because That's A Good Thing'.
Immigration does fix two particular problems though, especially targeted immigration of skilled workers rather than refugees:
1) It balances an aging, shrinking population which usually stifles innovation and depresses wages (both of which are happening in Japan right now).
2. It increases the talent density of industries. Talented people exist in every country and culture in the world. Attracting them to your country with a wage higher than their home wage is a good way to snowball a competitive industry (because these talented people make things successful leading to more opportunities for more talented people).
where in Western Europe is this currently happening? Which cities?
and when all the other cities in UK, France, Germany, Sweden and other "wir schaffen das" countries change as dramatically, the people who are currently saying "it isn't happening" will change their tune to "it's actually a good thing"