Perhaps they could video the cockpit (and the fuselage for that matter) and destroy the footage once the plane has safely landed. There could be streaming capability to the ground and if the feed is accessed, the pilots and crew receive a notification. Any unauthorized breach would be detected immediately. In the case of Germanwings, ground control would have been able to see what's going on once they detected the loss of altitude. It stifles me that in 2015, a young troubled copilot can end 150 lives in a way that can easily be prevented with simple technology.
While I agree a video would be useful in some cases I do agree with pilots there needs to be a balance between having information in a crash and creating a permeant record of what happens in the cockpit. Something similar to the flight data recorder where data is overwritten on a periodic basis might be a good compromise. Even so, a video record probably won't add that much information since things such as switch positions, throttle settings, instrument readings etc are already being recorded. Unless something unusual happened, such as with Germanwings, you'll basically just have a video record of who did what your audio and telemetry already says. One question is the cost worth it? Adding a few pounds of weight costs a lot of money over the life of a plane and that also needs to be factored into the equation as well.
As for preventing the Germanwings crash, how would technology such as a streaming videocamera prevent that? The pilot clearly trusted the copilot enough to leave the cockpit so all you have that that point is a video of what is going on but no way to prevent it. The type of technology that might have prevented it, an electronic medical record with automatic notification of employers when a doctor prescribes something that may indicate a lack of fitness for duty or deems a patient unfit for duty might have worked; but that would add its own set of problems nit the least of which is people would stop seeking treatment for conditions that they think could cost them their job.