It is also not true that no progress has been made in the understanding of String Theory in the last 40 years and it still seems like the best bet that could eventually generate a fundamental theory. What is missing is still a lot though:
- We don't seem to possess the correct mathematics to develop a non-perturbative formulation of String Theory and there are too many potential string backgrounds that we could expand around.
- It is also hard to derive the matter content of low energy effective actions from most brane configurations.
String theory provided major insight into non-perturbative quantum field theory as well. There is tons of examples, let me highlight one of them: The discovery of the Amplituhedron (Arkani-Hamed et al., 2012) was preceded by the discovery of the BCFW recursion relation (Britto et al., 2005), which in turn was motivated a relationship between perturbative Yang-Mills theory and the instanton expansion of a certain string theory in twistor space (Witten, 2003).
No, it doesn't. The original Standard Model from the 1970s did, but then neutrino masses were discovered and the Standard Model was modified to include them.
I have no firmly held answer to the question of whether it is.