Its a beautiful system but where my head explodes (and has been exploding for 4 decades) is over the following scenario. So in Johnny's system, I assign 21 to automobiles. My VW van gets 21.1, my Citron is 21.2, etc. and the insurance for each car gets a .8 so 21.1.8, 21.2.8, etc. And I assign 13 to Money. Insurance belongs under money so 13.5 is insurance and life insurance gets 13.5.1, E&O insurance gets 13.5.2, et…
A flat structure seems less organized, since you are “mixing” stuff, but as long as there isn't too much stuff inside, going through stuff one-by-one is faster than you think. If I do have a lot of stuff in a section, I either split into several sections in the top structure (so 13 is life insurance, 14 is other...), or go one level deeper (not preferred, but I do it when it's very clear and there is too much stuff, like photos, which btw sorting chronologically works best for me).
It is really not much of an issue having 50 top sections. It makes the organization transparent, and indexing, sorting and going one-by-one remains easy.