Sorry but thats not how physics works, gravity makes thing go down, so string goes down, the edges are being hold up hence the edges don't go down.
Your logic says since the string doesn't stay flat, the earth isn't flat, we are in the inside of a sphere.
Also about occams razor, what is ment by simplicity of a theory, looking at the complexity of the whole theory is wrong, because then a theory that can explain the most things becomes the worse theory. You have to instead compair the simplicity of explaining a thing both theorys predict. Once DET has any equations to predict the motion of any object in the sky this can be done, untill then occams razor doesn't apply.