Could string theory be an effective theory? 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.
 A: Considering scattering amplitudes it's commonly believed that string theory is UV-finite, so it gives you the exact answer that you are seeking, at whatever energy level, without needing approximations (at least in principle, in practice we know mainly perturbation theory). Intuitively, the UV finiteness is due to the finite size of the string. To be clear, there isn't a formal proof of these statements. 
However the full non-perturbative picture is still unclear. For instance the IIA superstring theory - and using $S$, $T$ and $U$ dualities all other superstring theories - can be seen as a $g_s\lll1$ limit ($g_s$ is the closed string coupling constant) of the eleven dimensional M-Theory. 
Indeed a bunch of $n$ D0-branes in IIA can be seen as a bound state at threshold (the branes are BPS objects, so repulsion equals attraction), with mass:
$$M=\frac{n}{g_s \alpha'}$$
This is the typical mass spectrum $M=n/R$ of Kaluza-Klein with radius $R$. So $R_{11}=g_s \alpha'$ is another dimension that opens up in the $g_s\ggg1$ limit, and the D0 branes can be seen as Kaluza-Klein particles of M-theory.
