Sorry, Kebrel, I didn't quite follow all that. You don't like the idea (which isn't Hawking's alone, mind you) that one cannot definitively prove a theory. That is, a scientific theory must always remain a theory and never cross over that great divide into absolute Truth. This is a key foundation of the scientific method; any theory must be discarded the moment that verifiable evidence suggests that it isn't accurate. If this were not the case, then we could well happen upon situations like Galileo was part of; astronomers attempted to preserve the old (Greek) theory in the face of contradictory observations. In that instance, the theory was given a higher standing than the evidence. Mind it, it isn't that the theory never correctly predicted the physical reality, but with more sensitive measurements its flaws became apparent.
As for String Theory, since you bring it up, it was always a dead theory for scientists because at no point did it meet basic scientific requirements (that is, at no point could the theory be tested). If physicists had been more concerned with what Hawking stressed in that passage (essentially, rigorous adherence to the scientific method), String Theory would have been discarded quite some time ago (only now is it leaving fashionable circles).
Advancement stops as soon as one believes one has reached a destination; if that destination can never be reached, logically advancement would never stop. Of course, that paradigm might shift someday as well.
But perhaps I misunderstood you somewhere?
