what references can be provided to back up the view that GA plays an essential role in the provision of commercial pilots? I really would like to see the evidence, because I have looked for it over the years, not just views.
exmpa

Circa 105 pilots out of 130 in the airline I worked for (large regional jet operator), GA was involved in their training – 86 or so were ex-flying instructors. Around 40 or so were still actively involved in the GA scene, either flying instruction on days off, owned/shared a GA aircraft or in the extreme case – compete in aerobatics.

In the 14 years I was in the aviation business I only ever came across six pilots who were taught by HMG. 3 x Rotary (RN), 2 x Rotary (Army), 2 x RAF (1 x FJ, 1 x V-Bomber). Ironically said ex-Victor pilot was heavily involved in the GA scene, before he started flying for us.

Granted nowadays, integrated courses are the primary tool for professional training, but for a number of professional pilots their first taste of flying comes from a trial lesson at their local airfield.

Then there is the fact that here are too many graduates from integrated courses chasing the small number of commercial jobs on offer, so some will inevitably end up as flying instructors at a small GA operator.