Did you read the post? This project does exactly that. (but focuses in IndexedDB for now because it's the only cross-browser thing that works. I actually tried a webkitFileSystem backend and it was slower)
Well, I woudln't call using IndexedDB "exactly that". As IndexedDB is rather fleeting. You don't use a server, that is correct. But IndexedDB goes away under many circumstances. Saving a file via the File System Access API would give the user peace of mind that it is safe. I did not see any mention of the File System Access API in your post.