GA flying is DIFFERENT to airline flying.
Try getting a pilot who has gone ab-initio - RHS Jet transport to convert to SP IFR in a Chieftain or Aerostar, put them into the GA world to operate a 30 year old recycled piece of ****e bugsmasher in all wx day/night and do hand to hand combat commercial ops and see how well they do.
It's a different way of operating.
On the other hand, I bet you the pilot with the solid GA background will convert to the Transport Category aircraft operation a helluva lot quicker than the other way around.
If you think that GA is only a place to learn bad habits, you don't understand it in the first place; and if you don't understand the value of a solid GA background, don't knock it.