The fundamental postulate of string theory is that matter is composed of tiny vibrating loops of string, and each vibrational mode of the string corresponds to a different fundamental particle. Now, since there exists infinitely many possible vibrational modes of the string, it follows that there exists infinitely many matter particles (fermions). Since string theory requires supersymmetry to be consistent, it follows that there would exist infinitely many bosons, hence infinitely many forces. Now, would a phenominologically consistent theory of ''everything'' be possible if there exists infinitely many forces ?
QCD is already a little like this. Mesons and baryons form families of increasing spin and mass ("Regge trajectories "). Nucleons are bound together by exchange of spin-0 pions, but heavier spin-1 vector mesons also play a role, and even tensor mesons of spin-2 and higher. The attempt to develop a theoretical framework which could describe the interactions of such families of particles is exactly how string theory was discovered. In fact, the mathematical structure of interaction (Veneziano amplitude) was found first, then people realized that this is how the excitations of a relativistic quantum string interact.
Also: the higher modes are increasingly heavy (because of the energy they contain), and exchange of superheavy strings corresponds to "forces" felt only over ultrashort distances (before they decay to lighter states). Nonetheless, they are part of what makes string theory different from other forms of quantum gravity at short distances, an aspect of what makes it work. So they're not a bug, they're a feature.