HH - way off topic now, but I disagree. All the private armies that were formed in WW2 (Popski's, Stirling's, the Chindits) were disbanded very soon afterwards. The Parachute Regiment and Royal Marines survived but were not then or now considered to be SF.
21 SAS was formed in 1947 as a TA regiment only and it was not until 1952 that 22 stood up, and that was due to the development of jungle warfare in Malaya and the need for a regular battalion.
If you look at recent conflicts it's clear that traditional military combat with defined front-lines and territory being lost/won is on the wane. Both SF and PBI need to adapt to survive in the new environment.