Let’s get it out of the way: she’s not against maximalism, old stuff, or necessary cruft.
The thing about Marie Kondo’s stuff that people don’t emphasize enough is that she’s not just saying “A thing must spark joy for you to own/keep it”, she’s also saying “You should maintain a positive relationship with all the things you own/keep.” If this is not obvious to you, I recommend revisiting how she talks about socks – it’s not just that you should fold them because it’s more practical than having a heap, it’s that they merit being treated with dignity. The transformation demanded does not only flow from the emotional (no spark!) to the physical (discard!), but from the physical (boring old toothbrush) to the emotional (imagine not having one – wouldn’t it suck?).
Really grappling with this in the context of the quantities of stuff you or I own can be exhausting. At the same time, there are arguments for that from the environmental (if you don’t love a piece of plastic enough for it to exist for 500 years and then become microplastic, should you bring it into your life?) and the social (all the garments you own were sewed by skilled workers from whom you are entirely alienated; think about why you can treat clothing casually). I don’t know that I, spoiled member of industrialized society that I am, will ever really live up to this demand, but moving in its direction seems positive.