This is a valid point. Some other theories that were originally thought of as radical and untestable were plate tectonics and germ theory.
On the other hand, I think the reason why this gets brought up so often for string theory is the degree to which it appears untestable. Multiple curled up dimensions and something like 10^500 possible versions of the theory, and with everything operating below the Planck length, which according to other widely accepted theories is the smallest possible measurement. How do you measure something smaller than the smallest measurable thing?
I think the only comparably untestable theory was atomic theory when first suggested by the Greeks 2000 years ago. Germ theory and evolution were relatively tame by comparison.
