I am not advocating for mass migration, I am saying that Japan does not have a cohesive society. Personally, I wouldn't put "the country" above its citizens, ever, and if these measures that cause great affliction and suffering in people are somehow justified, I don't hope for you to ever have to live in such a society. I'm not trying to be passive-aggressive, I'm honest here: the West already has a big problem with social normalization and the inherent contradiction of hyper-individualism and flattening consumerism, reducing personal expression and individuality to consumption of goods. The East takes this, multiplies it tenfold with their extreme consumption of fiction, pornography and gambling (jp gacha that don't plan to expand to global regions are infinitely more predatory than our regulated ones, same goes for korea), which are mechanisms to dull the spirit and ensure mindless participation in society, and inserts it into a brutal machine which comprises inhuman school hours (an extremely notions-oriented system that occupies 10 hours a day of the student's time if not more, adding their cram school culture), job hunting, work culture, and extreme social pressure. It's not that people are in a cohesive, organic society, rather they are exploited and given some wways for their dopaminergic system to not completely collapse.
It is dishonorable to fail classes, get average grades, not dedicate your soul to your job, not getting married and having children (which is borderline impossible in the society that they imposed on japanese people); school institutions post everyone's grades online, so that everyone can see them, and frequently it is reported that bullying and shame occurs due to not having good grades; black companies exist unchallenged; there is 0 space to stand out, which doesn't mean to jestermaxx and troon out, but to express in artistic means, have irregular hobbies, hold certain beliefs that contradict the mainstream, eat, drink, or walk different than others. Kyoutou's famous for the extreme passive-aggressiveness with which people address others, in a way that doesn't break tatemae but is socially recognized to express a particular sentiment that is antithetical to the surface interpretations (such as "what a nice watch you have" to mean "you've been here too long, get out now", or "your children are very lively, aren't they" to mean "they're fucking loud as shit and they're annoying as hell"), even in restaurants (offering ochadzuke to mean that the customers have eaten so much that they can only serve that, meaning gtfo). The rampant porn industry, the high school girls' panties vending machine, these aren't indicators of cohesion in my eyes.
Finally, no, it evidently does not help the country operate and run very smoothly, because of the phenomena that I have outlined in my previous post. Post-1991, Japan never could recover. Yeah, they had en economic boom during the preceding decades, but that doesn't mean anything; an underdeveloped, war-thorn country can rise up fast (again, see Italy with the post-war economic boom), but the moment a crisis hits, Japan never managed to recover not even partially so. For most of history, Japan has been a feudal system under emperors and then shogunates, but in that same time period, Europe was thriving, and eventually gave birth to those ideals of freedom and whatnot that permeate society and which have multiplied their success. In contrast, Japan's mentality virtually does not allow them to graduate, because they have been dominated by such an oppressive system for so long that either they internalized it, or they kill themselves from the pressure.
And regarding the "monoculture", I'm sorry but it's a bit different. One thing, imo, is organically developing a culture, see historically in England for instance; one thing is to impose it through conquest and statal mechanisms (european equivalents would be Germany in 1871, Italy in 1861). Japanese Shintou is very, very different from the native animist cults (and is in fact an artificial construct to promote the ideal and image of a unified Japan); Buddhism is imported from China and Korea. The State, through its means and through the fundamentally not cohesive Japanese society, oppresses japanese people all the same; mass migration would bring about the same problems that we see today throughout Europe, but again, I'm not advocating for that. Simply stating that Japan is not this amazing ethnostate you seem to think it is. (In fact, what we refer to as "Japanese man" is the result of intermingling between Joumon, Yayoi and probably some third wave in the Nara period of so called "Koufun" people.)
(and I haven't even begun talking about keigo lol)