You seem fixated on the idea that OPFS is some kind of ramdisk. It isn't. When a website stores a file in OPFS, the browser writes some kind of opaque data structure describing all stored files to disk. That data structure can take whichever shape the browser desires excewpt for just dumping those files in a directory in order to isolate OPFS from the regular filesystem.

You can query the browser for the maximum quota available to you and then just tell it that you want a file that big. Boom, now you own that chunk of the user's SSD.

As has been pointed out elsewhere, that's still of dubious value for fingerprinting but I don't particularly enjoy the thought that random websites can just occupy gigabytes of space on my computer without even asking.