'Quiet luxury' has been around since (at least) 2014, at the time when there were still minimalist 'fashion bloggers' and it has been 'in' since then in niche circles. Some of it came in the form of normcore, other in the form of more elevated minimalism, it's just that some idiotic TikToker suddenly decided to rename it two years ago and the whole fashion industry followed (read Vogue, etc), because it was a catchy name that happened to bring sales. Tiktokers are renaming nowadays every single style that previously existed while they were wearing pigtails for school for the sake of going viral. What is upsetting is that people actually fall for the (click)bait. Minimalism has always been about having timeless, high quality clothes over quantity.
I know I'm going to sound like a conspiracy theorist, but I believe that due to the economic crisis that has been unleashing since the pandemic and due to an economic system that requires from companies to endlessly grow, fashion conglomerates have decided to increase profits by making fashion 'simpler', definitely NOT to be more 'sustainable', but in order to save on materials, pattern making, etc, that's why it's so convenient now more than ever that 'quiet luxury' becomes a global trend. The latter is why I also believe it won't go away anytime soon.
Quiet luxury can be both a good thing and a bad thing. It can be a good thing if you are not a fashion person and will just stick to timeless high quality basics. It can be a bad thing if you 'invest' in an Agnona sweater, yet you never wear it, but instead still wear your sequined sweater multiple times more. Sticking to whatever you wear the most being it printed, textured, or whatever distinctive non basic or basic resonates with you is the most realistic approach (and most sustainable too), simply because 'capsule wardrobes' and 'basics' are a marketing myth absolutely detached from what people actually really wear daily. Lastly, the worst problem with quiet luxury is that it is a nondescript type of fashion (not arguing that it can also ironically be some sort of status symbol), that turns everyone into a clone, when expressing your individuality through fashion is a form of freedom.