Snarek:
CPL = paid for flying.
PPL = Private; not aerial work, not Instructing not charter - Private only. Recreational/pleasure & OWN business purposes.
If a 500 hour Private pilot feels it's time to take the small step into earning some income for their skills, it's just a small step to a basic CPL.
For a 500 hour Private Pilot (lets assume there is 100 command in there somewere) it's not that difficult to get the CPL.
Possibly a few flights, a few examinations (a lot easier these days) and the Private Pilot will then have the legal right to receive $$ for their skill & efforts.
After that, the now legal, ex-PPL can get an Instructor Rating, etc. etc.
Failing that, the hypothetical PPL Instructor would not be able get paid - period.
FREE INSTRUCTION: This would stuff the already dim prospects of the legitimate career CPL.