Very interesting point. One could envision such a system being created, then acquiring mostly benign information up to the point of the very rare catastrophe, at which point the parameters we really need (when reconstructing events or faults in a post-hoc fashion) are not acquired due to electrical catastrophe on the vehicle or in the immediate environment of a thunderstorm.
NASA was able to glean a lot of information after the Columbia and Challenger accidents from data telemetry but those were obviously densely instrumented vehicles with dedicated data acquisition facilities groundside. The bandwidth and storage issues with trying to do the same for hundreds of routine passenger flights would be staggering IMHO.
