So can you implement Folders with it?
To be that guy for a moment: well, hackchewally…¹
Directory entries do take up space in the MFT, but that doesn't show up in explorer which is only counting allocated blocks elsewhere. You will eventually hit a space issue creating empty directories as the MTF grows to accept their allocation.
You can do similar tricks with small files. Create an empty text file and check, it will show 0 bytes size and 0 bytes on disk. Put in ~400 bytes of text and check again: explorer will show 400 bytes in length but 0 size on disk because the data is in the directory entry in the pre-allocated MFT. Double up that data, and it will be big enough that a block on the disk is allocated: in properties in Explorer you'll now see 800 bytes length and 4,096 bytes (one block) on disk. Drop it back to 400 bytes and it won't move the data back into the MFT, you'll now see 400 bytes length, 4096 bytes consumed on disk.
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[1] though don't let this put you off enjoying the splendid thing overall!
Thank you for elaborating the technical details though (plus I did not know the small file trick, that's a neat bit of trivia).
Drop it on youtube or tt, get your popular friends (this might scupper me, if anyone I know is an online influenza they have the good sense not to let me know about such proclivities!) to make a review of it, and see how far and wide it spreads with people either in on the joke or idiots just parroting it for views.
In the Unix land, the same sort of thing exists with file systems such as ext4 and UFS2: they also depend on predefined regions for metadata. If you'd like to venture into ZFS, however, (almost) all metadata is dynamically created and destroyed on an as-needed basis, and as such, ZFS always reports the on-disk usage by counting both user data and metadata. It's easy to see a directory grow to many megabytes by just creating a lot of empty files inside of it.
Not quite, as the MFT can't consume all the device, so you can't fill the whole lot that way, but you could cause significant inconvenience that is not undoable without migrating to another filesystem. That migration might be possible without a reinstall as such, though it would take a lot of manual jigger-pokery (and I don't know of any tools that would help, it isn't something I expect someone has felt the need to write tools for) so the reinstall would likely be easier.
> have viruses been known to exploit this?*
I doubt it. If the virus is intended to extract something from the user (exfiltrating data, ransom, making them part of a bot-net, etc.) then it wants to work in the background not causing inconvenience like that (until it fires in the case of encryption & ransom, but that is a different sort of inconvenience), and if the goal of the virus is just to cause a mess then there are more effective methods of doing so.