The point of divergence lies in X-1's engineering rather than a sudden change of doctrine. Her camshafts, gearing, and drive arrangements attempted substantial technical advances alongside the conspicuous gun armament. Historically those systems made her a maintenance burden and frequent dockyard occupant, preventing the Navy from learning through an operational class. A reliable boat would instead give the Royal Navy a plausible way to answer its cruiser shortage while other powers were also exploring large submarines.
Notable Point · 1:06-2:24
Reliability is the hinge of the X-1 counterfactual
A dependable X-1 could turn experimental machinery and a British cruiser shortage into the start of a sustained cruiser-submarine development cycle.