I’ve always been amazed at how much faith physicists have in the principle of least action, which says that all classical objects travel along a path that minimizes the action. In quantum mechanics, particles will take all possible paths but the amplitude is weighted by the classical action along the path (so the most likely path will be the one of least action). Steve Hsu discussed the path integral, which sums over the paths, in his blog recently.
From what I’ve seen as a spectator, “Theories of Everything” have fiddled with symmetries, spatial and temporal dimensions, been comprised of strings or branes or what have you but they’ve all been based on the premise that the quantum amplitude is giving by a sum over paths weighted by some action. I’ll be the first to admit that this approach does work amazingly well for all the energy scales that we have tested thus far. Quantum electrodynamics is the most accurate theory we have.
However, we should not forget that least action and it’s quantum cousin are assumptions. They were discovered empirically. Yes, in order to make any progress we must make assumptions but sometimes these assumptions are so internalized I don’t think people even realize they are making them. The greatest breakthroughs often involve questioning our basic assumptions. Relativity, quantum mechanics, plate tectonics, democracy, and so forth all arose as fundamental challenges to the prevailing status quo. So even if M theory turns out to be the right theory some day it still won’t tell us why we have the principle of least action.