the US economy has boomed largely in part due to money printer brrrrrrrrrring for a very long time. it isn't particularly booming right now with no ZIRP.
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'.
The US economy was booming before the ZIRP began though, and that was on the strength of the technology industry.
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).
London, Birmingham and God only knows how many other cities in the UK. London in particular is the most obvious example of why accusations of paranoia and alarmism are either malice or stupidity - the city has changed from being over 95% aboriginal to <50% aboriginal in just 40 years. and it didn't even end there - now it's below 40%.
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"
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'.