Right, the spy threat would be the "security reasons" I guessed at. But still, wouldn't successful projects which were later decommissioned be more at risk of

Right, the spy threat would be the "security reasons" I guessed at. But still, wouldn't successful projects which were later decommissioned be more at risk of spies than an unsuccessful project? Yet successful projects do not have their blueprints and airframes routinely destroyed without a trace.