Former ITA engineer here. Our airfare search product QPX was untouchable at the time due to design: it got results that were far better than those of the competitors because ITA was modeling the problem better (search through a graph). While competitor tech debt didn't hurt us, I don't think it was the pivotal factor in ITA's success. As you point out, our hopes of replacing a major carrier's reservation never came to fruition, unfortunately. A res system is a complex beast.