it's just about fears and habits. Habit is nothing, I have already abandoned many nested genres. But there is a concern that after placing all the files in one folder, I will not be able to find anything without musicbee.
Another problem is tags. I have 40,000 tracks (not all of them I need, I plan to rethink my methods of collecting music and get rid of what is not fun) but a smaller part of them with tags. This is a very long job. Even if you use MusicBee tools or other tagging services, it is very long, because there are a lot of wrong genre tags on MusicBrain, Discogos. Everything needs to be manually entered, I browse my 2 main sites, where I get information about tags.
On top of that, this tag confusion in MusicBee is splitting the same album apart. It is very uncomfortable. See when the album Album 1 contains tracks from 3-10, and elsewhere the remaining 1-2 tracks.
The advantage of storing in one folder is that it doesn't matter what subgenre the artist has. If he's in progressive metal, then he has something to do with it for sure.