There was some truth in that - many lines were in a dreadful state by the end of WW2, so even some minor lines needed renewal - though sometimes with second-hand materials.
On the other hand, busy main lines often needed renewing every 10-12 years, so parts of them might have been relaid a couple of times in FB rail by the rail blue era, with no risk of closure.
The decision to switch to FB rail was made very early in BR days. Some of the big four experimented with it in the 1930s.