You could argue that one of the most famous proofs of last 30 years - proof of Fermat's Last Theorem used such approach. Andrew Wiles had proven that if this FL

You could argue that one of the most famous proofs of last 30 years - proof of Fermat's Last Theorem used such approach. Andrew Wiles had proven that if this FLT was false, then certain theorem about elliptical curves (unrelated at first glance and from different area of mathematics) would have to be violated, and mathematicians already knew it was true.