The issue goes back years - IIRC the original opposition was a well founded fear that the company would search any footage in order to nail something onto the Flight Crew - even if there was no incident. The crews, in the days when cockpits were not "sterile" and there was a great deal of chat, banter and a lack of adherence to many procedures, knew they were handing a loaded gun to people who were not their friends.
These days cockpits are much more controlled spaces, and people (well, almost all of them) no longer indulge in unacceptable behaviour on anything like the scale in say the 50's & 60's. We should be able to come up with rules that allow a camera and rigorously control the uses the film can be put to. Maybe a Union rep in the loop?
I find it odd that almost every Taxi & Uber I take to the airport has a camera (s) but the aircraft doesn't