Seems the most natural course of action of any to me that the makers of a new, advanced piece of defence equipment were asked to destroy everything associated with it when it was cancelled.
The alternative would have been to keep all jigs, models, blueprints etc etc under secure, guarded, audited conditions, indefinitely. Remember this was a time when the airliners from the East looked (un)surprisingly similar to their Western counterparts.
Shame, but no conspiracy, just plain good economic sense (unless you are an aviation enthusiast!).
And don't forget, the US are quite happy to assimilate other people's hardware if it's better than their own (Canberra, Harrier). Much easier than putting pressure on an Ally to dump a project (giving a left wing govt perfect anti-US
PR at the same time...)
Imagine how good the Tornado would have seemed now if they'd cancelled that at a late stage....