I know that many quantum field theories could be low-energy effective theories in String Theory (ST), but I've also read and heard that ST cannot itself be an effective theory.
I suppose this has something to do with the UV behaviour of scattering amplitudes in ST and its conformal properties. Is it known to be impossible, though, for ST to be a (e.g. low energy) limit of another theory? If so, why? Is there merely consensus amongst experts or is there a rigorous proof?
I can't see of any motivation for considering ST to be an effective theory, since it's supposed to explain everything without any adjustable parameters. I'm just curious about this point.