Tube Technology like all technology is a dog that had its day, but more importantly from the Hey Day the Tech' was all about the people involved in the production.
It is safe to say the Tube was replaced by the much less power hungry Transistor and the Transistor by its function is quite Tube like when used in a audio circuit, as in its earliest guise it is a not too complex simi conductor, it has limited functions to which it is dedicated.
The Transistor was already mainstream in the 1950's, so the idea of Valve Production Plant's trying to compete was most likely a commercial suicide, the writing was on the Wall. n
Where the real changes are most likely to have occurred, is a result of War in Mainland Europe the Factories that had brought on the skill sets to be creative and innovative were losing their employees at a fast rate to the War Effort and as a story of tragedy, not many were to return, and plenty that did return, may not have been able to fall into life as it was once known.
Factories were pretty much set back and as they were trying to get back to their pre-war status, the Transistor was fast coming through as the more attractive option.
The Wars of the Twentieth Century, in conjunction with the follow up Arms Races are most likely why there is such a history over the past 40 Years of NOS, as most Tubes are from a substantial intinary of a Military Surplus released into the mainstream.
It would have been wonderful to have been blessed with the all seeing eye and have bought a Batch of Mullard ECC32's in the 90's, when they were a few £'ss each.
The 32's are quite cheap in comparison to some other Brands, that not too many years past were very cheap finds.