I identify with Brewster here. A unified theory of quantum gravity would not be a scientific breakthrough.
Philosopher Massimo Pigliucci partially explains what is wrong with Sean M. Carroll's justification of untestable theories.
He is way too easy on Carroll. String theory and the multiverse are pseudoscientific because they do not make testable predictions. They are not falsifiable. He says that they are "speculations somewhat grounded in well established physics" but they are metaphysical speculations, and the well established physics gives no reason to believe the speculations. The Popper notion of falsifiability works fine for dispensing with these ideas as unscientific.
Update: A comment on a new relativity book says:
“Maybe there’s a lesson here for some of today’s string-theory sceptics?”Yes, that is right, and that is why this blog pays so much attention to the history of relativity. If the story is going to be used to justify all sorts of other endeavors, then it is important to get the story right.
It doesn’t matter how empty an argument is, you can always count with someone comparing it to Einstein and relativity. That’s a rule that works both with cranks and people with reputation.