Al R - National Service turned all three services into huge training machines, turning over everyone on NS every two years. It was therefore possible to double the size of the military overnight with people who had been out for less than two years. While they would have lacked currency, they'd have been fairly useful very fast, and that was the need during the cold war.
Now we have a much bigger problem trying to man-up for anything. Retention may sound like a good thing, but if you keep people for so long that you don't train many new people, then you can't expand very fast. The capacity of the training system can quickly become a limiting factor.
In the late 70s the RN took RW pilots from their first day at Dartmouth to their first front-line ASW squadron in about 27 months, as there were no hold-overs.