So we know that there are 5 types of string theories (Type 1, Type IIA, Type IIB, $SO(32)$ heterotic, and $E_8 \times E_8$ heterotic). It was shown that these 5 types are just limits of something called M-Theory, which we know exists but we do not understand what it is.
Given that we know string theories are simply limits of M-Theory, does this mean that string theory isn't the complete theory of quantum gravity? (Just like how Newtonian Gravitation is incomplete but it is a limit of General Relativity; or how ordinary quantum mechanics is incomplete but is a limit of Quantum Field Theory). If so, why are people investigating string theory when there is M-Theory, a more complete theory, to investigate?
Best Answer
String theory and M-theory are not distinct and certainly far from being complete. Even though M-theory attempts to show that the five string theories are smaller parts of a larger concept, it's important to note that there is still much work that needs to be done in demonstrating that strings are an accurate description of reality (indeed no one can be certain if the microscopic world is composed of strings, to begin with). Indeed it may even be the case that future research in string theory confirms that current M-theory may need to be refined or even drastically changed.
Your string theory to M-theory analogy with the limiting cases of Newtonian gravity and QM going to general relativity and QFT respectively, is perhaps not a fair one since our understanding of the Newtonian/classical world and the low energy quantum world is very good. This is why theoretical physics research is active in the world of particle physics and cosmology, and not usually in classical physics and non-relativistic QM.
Basically, that we have M-theory shouldn't mean we abandon research in string theory because string theory needs much more research to confirm if it is a good description of reality.