Why are pilots so against it?
Anything to hide?
Trains, buses, ships, subways even cabs have cameras on it pretty much anywhere in the world for years and no one complains.
It is about safety gentlemen, sorry if this offends your valuable privacy, you're not better than any other worker subject to this.
Can you name a safety benefit of having a camera in a cockpit?
Can you think of any accident in recent times that may have been avoided with a camera in the cockpit?
Everything has a cost/benefit ratio. For operators and manufacturers, a camera system is one more expensive thing that has to be designed and maintained. One more thing liable to get an airplane AOG when it breaks. A few more kilos of hardware sucking more of your fuel.
Unless someone can demonstrate a clear safety benefit from having one, all I can think of is making the investigator's job easier and a assuage the morbid curiosity of a bunch of internet armchair enthusiasts.
Another aspect people outside of the profession often fail to grasp is the potential legal implications.
Any safety event in aviation is very quickly followed by an army of lawyers erecting legal walls around all the parties concerned. This is where you'll usually find a billion dollar aircraft manufacturer and their battalion of lawyers going against a billion dollar airline and their platoon of lawyers going against a couple of pilots who will have to scrape whatever few savings they have to find a couple of small firms willing to defend them.
The former 2 will always try to pin the blame on the pilots, they're easy preys.
There is a reason manuals are essentially written by lawyers nowadays. Whatever happened during the safety event, lawyers will always be able to find something among the thousands of pages of ops manuals that the pilots were not compliant with.
A camera, aside from not increasing safety since there is nothing it can do to prevent an accident (unless you assume pilots would somehow fly safer with a camera pointed at them), will only give more fodder to lawyers.
Not to mention that it's bad enough that the crew's last words essentially end up in the open for all to listen to, I can't imagine having the footage of their last moments alive be freely distributed online for all those sick enthusiasts of macabre content to watch, since there will essentially be no way to ensure the tape isn't leaked. Great for the families...