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In our commercial world you get paid what you are worth. If you are not getting enough have a look around you. There will always be suckers who will spend lots of money and effort to fly anything, even for no pay.
While GA is just a training ground for airline pilots there will always be a plentiful supply of hopeful, desperate, broke newbies. An oversupply. So their services will be cheap.
This is now spreading to the regionals and the majors. It will take a while to make major changes to the major airlines, bu the effects can be seen already. Some of their T&C's are being eroded.
One solution would be to change the legislation so that much more multicrew training and a multi crew licence is required for airline operations (as an F/O) with little requirement for GA hours.
That way there would be two streams of trainees and each would be training for the appropriate skills for the operation they want to go to. And GA would not be polluted by people who do not want to be there. So there would not be the oversupply of desperate CPL hplders who now cheapen the GA T&C's and pull down the T&C's in the regionals, and the majors.