Japan does not have a recent experience of high levels of emigration. (It did back when Japan was still a poor nation. There is a large Japanese diaspora in the United States and a few countries in South America, notably Peru and Brazil. That's a long story.) Factors counseling not emigrating include linguistic isolation (the overwhelming majority of not-comfortably-middle-class Japanese people do not speak useful levels of, most relevantly to their chances for successful emigration, English), cultural attitudes regarding the importance of Japan to Japanese people [+], and similar issues.

Your second question: depends on the cashier.

Note that a good portion of the things I were referring to are a result of falling out of the class you were born/raised/worked into. (Japan doesn't have classes... but Japan totally has classes.) If you never expected anything in life other than going to an agricultural high school and maybe working on the farm, like many young'uns in Gifu, then moving to Tokyo and working at a convenience store might not be catastrophic. If you expected to test into a good college, and did, and then end up working at a convenience store... that's a violation of the proper order of things.

Also, I would give non-zero credence to an explanation that a hypothetical observer who can't speak the language and has very little experience with the culture perhaps not understanding subtle distinctions like the difference between one's participation in a public social ritual and what one can tell one's closest friends when all of you are drunk. What's a good US example... hmm... OK, here's one: sincerity of religious belief. That's a spectrum, right? And Americans exist along that spectrum, right? And we have to have functioning radars for where someone is on that spectrum or it's socially disastrous, right? Imagine a hypothetical politician who pings your radar as "Absolutely zero sincere belief in any religion." Think backwards to the cues they give off to give you that impression. OK, now imagine that guy says, in response to a debate question, "Mr. $FOO, what is your favorite Bible verse?" and answers "I really like all of them, because I'm a Christian." That probably changes your radar value not at all, right? Because that's exactly what you'd expect a politician to say? The modal Japanese person, who probably does not have as good a radar on this issue as you do, will probably be confused as to why you think this politician is not religious when he just said something religious and claimed belief in a religion. And you could imagine writing a fairly long anthropological comment on a message board to that person saying "No, no, that's just the way we do things here. That question always gets asked and not having an answer to it, or disputing the basis for the question, would be political suicide. He's pretending to care about it and we pretend, for form's sake, to believe him." whereupon the Japanese person would say "Wow, you crazy Americans, so inscrutable."

[ + ] "What, like nationalism?" Not exactly the sense that I meant it in. Try on your mental model of a nationalist American for a moment: suppose an American spends several years abroad then comes back. Is this happy or sad? Happy, of course, because America is clearly the best place to live in the world and of course Americans would prefer to live in America, like everyone else would if they had the opportunity. A Japanese nationalist, on the other hand, would more typically say that the returning Japanese person is a sad event because the traitor will probably bring their foreign contagion with them. Is there even a word in English for what I'd be if I moved back to America next year? "What, an American?" No, a word which describes "previously lived a long while in another country." Japanese has a word for this (slight oversimplification for brevity going on here but I'm thinking of 帰国子女) and if it describes you you have challenges associated with that.