People talk about it all the time, especially on toxic forums filled with foreigners opining about Japan like r/japan. There are many reasons why immigration is not the silver bullet you think it is (some of which have been given by other replies). But the important thing is that Japan and the Japanese get to decide where to take their society and no one else. Telling the Japanese what they should do is arrogant paternalism.
People are allowed to have opinions that something would be a good course of action or not. That's not paternalism.
Do this, be more like (supposedly virtuous) X and don't be Y & Z is clear paternalism and, in my opinion, highly arrogant paternalism to boot.
What's far more arrogant is you thinking your culture is so magically different that you're somehow immune to such criticisms. I wonder how the Koreans and Chinese would feel about that.
It’s okay to admit that the US has done immigration well. It has. There are a great many things that Japan does better than the US. We can discuss them without taking umbrage.