How submarines might develop without the Second World War

How would submarine warfare develop if the Second World War never happened?

Along a similar path but much more slowly, with AIP, fast submarines, and nuclear boats arriving later.

Submarine technology would continue developing in the absence of the war, but on a later timeline. Routine air-independent propulsion might arrive in the 1950s, genuinely fast submarines in the 1960s, and nuclear boats in the 1970s or 1980s. The sequence remains recognizable because the underlying engineering incentives still exist. The missing wartime urgency would slow experimentation, production, and operational learning.