Any time it is suggested that something be automated (such as release of the FDR/CVR from the aircraft) - one always has to think about what would happen if the automation burps and causes an incorrect event.
All automation can fail, and even safety locks on safety locks can fail or be deactivated through error (cf: accidental (and fatal) wing feathering of SpaceShip Two).
So what happens if your system decides to release the FDR/CVR and float into the slipstream during a normal cruise at 38,000 feet and 480 knots? Does it jam or rip off a control surface or fin? Does it compromise the integrity of the pressurized cabin?
Engineering is not "I have a good idea!". Engineering is "I've had an idea - and have investigated every possible failure mode - and can provide evidence that none of them will have a fatal outcome, when (not if) they occur."