@Kaarme
You're fine with one expression of cultural loyalty, but not another? The onsen is no different than importance on the katana in this setup. It'd be the same for anything else associated with another country in that country's fiction.
It'd make even less sense for Japanese comics made by Japanese, sold to Japanese, with a Japanese main character, to praise things not of Japan over those homegrown expressions. In fact, the only ones I see doing that tend to be extremely chuunibyou, specifically absolute westaboos. The sort who adhere to American gunsmithing and German naming schema. And that's meant to be played as a foil to what's considered normal and acceptable in Japanese society.
What mystifies me is your opinion is not an uncommon one. People are fine with literally anything else Japanese-oriented being praised or uplifted in manga EXCEPT weapons. The moment a katana is featured as an important weapon people start complaining that it's not some sort of historical Western weapon that's never even crossed blades with a katana before, let alone been something a Japanese main character would not have grown up with legends surrounding. I remember another isekai demon king manga where the guy received a magical katana and suddenly people are complaining that it's not a poleaxe or some other specifically Western halberd. I don't quite get it.
Would they have been happier if it was the traditional 'longsword' (arming sword)? That's even more cliche than the katana, it's literally the blade of the fantasy genre. You will see it always and forever if you take any interest in fantasy - it's boring. But other, less common Western weapons, why would they appear?