There is always the problem, when positing "WI Inventor X had never existed" PoDs in modern industrial societies, that if there is a demand, particularly by the government for military technology, and the ability to pay for it, it'll probably be fulfilled sooner or later. IDK if AJ Higgins' experience was irreplacable, but the fact that literally hundreds of scientists worked on the Manhattan Project, the jet projects in Britain and Germany, and on creating ever-more-advanced tanks shows that usually, even the most esoteric designs were not the products of individuals.