If the police can't respond with an appropriate amount of force to make it impossible to destroy all of the attending officers cameras, they are not doing their job correctly.
Also, if the body camera isn't (1) robust enough to withstand an attack; and/or (2) streaming or at least storing the footage quickly enough to capture evidence of an attack, then whoever designed the body camera system has done an unbelievably awful job.
They should still be able to store the video up until it was destroyed which would show the destruction.