And what about the Jewish law in the Shulchan Arukh that one may not leave the Land of Israel for abroad just for a trip?

There is a lot of room to be lenient about that. You can find surveys of the issue online. https://ph.yhb.org.il/06-03-09/

What Rabbi Melamed wrote seems reasonable on its own logic, and I also remember many years ago, the first time I went on a trip to Switzerland, that beforehand I heard a lesson from Rabbi Asher Weiss, may he live long, with a leniency in that direction. But what is difficult for me is that there is another halakha in the Shulchan Arukh regarding the permission to get a haircut on Hol HaMoed for someone who returned from abroad and had no opportunity to get a haircut beforehand, on condition that he did not go abroad for the sake of a trip. And there the Mishnah Berurah and others say that the reason is that it is forbidden to leave for abroad for the sake of a trip. And there it would be very hard to fit in this nice distinction between a long trip and a short trip, especially since it would have practical halakhic implications for that specific law: that someone who went on a short trip abroad and returned on the eve of the festival would indeed also be permitted to get a haircut on Hol HaMoed—really the opposite of the plain law there in the Shulchan Arukh. That already feels like too much to load onto my poor soft stomach, apparently.

I don’t see there anything different from what I wrote. He says that if someone left in violation, he has no permission to get a haircut, and a trip is a violation. Once one reaches the conclusion that there is permission for a short trip, it is no longer included in that rule, and in any case there is no problem getting a haircut.
Beyond that, even if you concluded that according to his view it is forbidden, the Mishnah Berurah is still not preferable to all the halakhic decisors who permit leaving for a trip—almost all of them.
However, the Magen Avraham and Be’er Heitev there wrote that they permitted it only when one leaves for the sake of a commandment, unlike the Mishnah Berurah, who wrote that even leaving with permission allows the haircut, and only a forbidden departure is what they prohibited. According to their view, it would seem to be forbidden. But one still cannot infer from their words that there is a prohibition on going traveling, of course. This is only a practical difference regarding the haircut.

One should remember that in their time, an ordinary trip was for a long period of time, since there were no flights.

The truth is, usually anyone who argues not only thinks in his mind that the truth is with him, but also wants in his heart for it to come out that way. Here, in my mind I hesitate, but in my heart my whole being wants the truth to turn out to be on your side, because otherwise we won’t know what to do with ourselves regarding all these trips that are publicized all the time from every platform, magazine, pamphlet, and newspaper, across all sectors, and it skips almost no group. Among them are great Hasidic rebbes who go to Switzerland and so on. What can we say and what can we speak of, aside from that holy man, Rabbi Chaim Kanievsky of blessed memory, who was detached from all worldly matters, yet knew thoroughly the street names and geographic map of Ashkelon, because once people got stuck there and didn’t know their way, and he directed them how to get there. And when those around him wondered how he knew, he said that he had learned it so as not to get entangled in leaving the Land for abroad when traveling to serve as sandak in all corners of the country, and for him this became Torah. And there is also the well-known story of some yeshiva boys who came to ask Rabbi Elyashiv of blessed memory whether they were allowed to travel abroad for a friend’s wedding, and he answered them: on condition that they not go sightseeing. How strict we Lithuanians are. Or maybe this is simply a decree that the public cannot—past tense—abide by?

To the last commenter: if what you wrote in the name of Kanievsky and Elyashiv is correct, the state of Judaism is even more dismal than I thought. Hard to believe that this is what you people are proud of.

On the contrary, it definitely does seem like something to be proud of. Even if I do not agree halakhically, if that was their view, it is very impressive that they were so careful about it and devoted effort to it.