As everyone else has written, two very different things are being mentioned n your last paragraph: First, the application should be just a directory on disk. I

As everyone else has written, two very different things are being mentioned n your last paragraph: First, the application should be just a directory on disk. I wholeheartedly agree, this should be the way to install applications, and OSX has shown that this works wonders. Second: it being limited to only read and write files in that directory. No, that's the wrong idea, the application should be associated with some extensions and should be able to edit documents in your Documents directory. It makes no sense otherwise.