Well US Gen Z have the least purchasing power of any generation thanks to being the youngest and also getting screwed by the inflation/job/housing markets so its not surprising that they would be pragmatic about how they spend consumer dollars. Boomers have the most assets and highest income (if still working) on average so its no surprise they can afford to flex the flag, whether they actually follow up on it is another matter.
While Korean companies are certainly getting hit by Chinese competition, it would be naive to think they won't adapt. Don't assume that just because you have the bigger market you will automatically steamroll the competition, people have been making that mistake about Israel for years and they are even smaller then Korea, yet they are home to one of the most high-tech and competitive roster of companies out there.
Last point, unrelated to economics that chauvinistic attitude toward Korean culture is totally uncalled for, regardless of any shared heritage they have evolved into their own culture that they are proud of, and as someone who travels alot and has many Korean friends, they are very good at marketing and branding given how many foreigners try to immerse themselves in the culture. While Koreans may be tied to China for the sake of general prosperity, if you demean/disrespect their culture you can bet on many unfavorable views and a desire to keep a reasonable distance despite your flexing of wealth.