In reply to Jeremy Cherfas.

I think her most valuable thesis is really the "only keep things which spark joy in you." I suspect that I'm just as messy/untidy as you are (perhaps worse) in terms of having things everywhere and not necessarily putting them away. The difference with her "method" is that you've first gotten rid of a magnitude of order of junk. Thus the new "mess" that surrounds you (even when things haven't been put away) is at least the things that bring you joy and make you happy. She does discuss in that one portion you mentioned that tiding up your own space first will likely help to encourage those around you to do the same--that old "lead by example" chestnut.
Another aspect of her thesis, which I haven't seen laid out anywhere else, is that it dovetails somewhat with some of the GTD (Getting Things Done) and Bullet Journal philosophies which tell you to write everything you need to do down on a piece of paper. This helps to keep your mind clearer of all the minutiae you might otherwise try to hang on to (and typically forget), but still know in the back of your mind that you're forgetting to do something, but aren't sure what. Kondo's piece helps you clear the physical clutter (compared with the mental clutter) out of your life.
Of course it all sounds lovely on paper. We'll see what it looks like a few days, weeks, months hence... I will happily report that after donating away a lot of clothes and refolding things, I've got several additional empty drawers which I can put tsundoku into.