lightbright
“What are the barriers…”
financial (primarily)
Even if the mashgichim worked for the agency whcih they often due, that doesnt really solve the problem. Say R’ Yankel is a mashgiach temidid and yukel’s knish gesheft. Even if the OU is the one paying him, if he pulls the hashgacha he is out of a job. It is not like there are restaurants waiting around sans mashgichim that he can quickly get a new job, and he wont get paid for not working.
Unless there was a communal fund to pay mashgichim whether or not they are currently supervising a store/faciility/plant etc ultimatly you have to relly that the mashgiach is trustworthy enough to forgo hsi own sallary (at least for a time) to prevent you from eating treif.
(There are some smaller chasidish hashgachos that do work this way)
that said
“If the plant manager does not like how the mashgiach’s certification then he/she can just fire him and get a new one who will do as he says.” I dont think that is quite right, if the plant is under the OU, regardless of who is paying the mashgiach I dont think the plant can just replace the mashgiach without the supervising agency’s ok.
“There was a Rav in Chicago…”
Ive heard that with the Ridvaz, though Al Capone must’ve been quite tough at age 6 for it to have been him