Whenever they add another dimension to Modern Day String Theory makes me think of how people used to build Geocentric models of the universe. The idea behind it was that if they just added enough circles, the math would work out and they would indeed have their geocentric model of the universe...
String Theory, as well as M-theory, are still largely theoretical, meaning it is provable mathematically, but currently experimentally and empirically untestable.
Plus there are number of competing versions of String Theory, as well as Superstring Theory.
For these reasons, that's why it is not a "scientific theory".
The whole purpose of String Theory is to unify Quantum Field Theory with General Relativity.
There are four fundamental forces (or 4 interactions) in the universe, and General Relativity only focused on gravitational force, as well with large body masses, like stars, planets, galaxies etc.
Quantum physics on the other hand, focused quantum-size particles (hence Particle Physics), and on electromagnetic force, strong nuclear force and weak nuclear force, but not gravity.
So String Theory and Superstring Theory is trying to combine gravity with particle physics.
I am no expert on theoretical physics, so I cannot say one way or another if M-theory is a dud or not. All I can say is that these fields of studies and researches are still quite new.
You have to remember when General Relativity and the Big Bang theory started out, these were theoretical physics too, until the observable discoveries made into "scientific theories".
The Big Bang started in 1920s, independently by 2 different physicists, Friedmann (1922) and Lemaître (1927), but it took science 37 years for them to discover the Cosmic Microwave Background Radiation (CMBR) in 1964.
It may just require time and technology for them to finally get the evidences needed to verify if String Theory is true.
