This is not true. As long as number of items per node is small adding a new subtree affects only few nodes so relearning is quick.
> the vast majority of people absolutely suck at hierarchical navigation,
Yes, I have seen those desktops with over 100 files. But I also observed that people navigate them relatively efficiently because they know where the items is on the desktop using the position on the screen and the icon as rather effective visual hint. They often do not want to introduce folders because common GUI do not permit change the folder icon with minimal efforts. And they hate big time when something changes the order of the icons.
But when the number of files growths, folders and even sub-folders do appear. They are very add-hock and sometimes named so the folder would appear on a particular place on the screen that person uses as a navigational clue.
The tag systems that I have seen completely loose that visual hint about item being at a particular place. For example, a tag system cannot express the hint "to the left of the picture of a dog".
As others already pointed out, tags are brilliant for searching and accessing rarely used files or files from other users, but for navigating big personal working sets one often wants a stable ad-hock structure that has nothing to do with file content but rather reflects personal habits and clues of working with files.