Re: similar looking aircraft
"The appeal of stealing details of the F-35 is that it will make it possible to screw not only its flight & fighting systems but also its logistics & maintenance systems"
That and knowing what it's weaknesses are. It's pretty clear that noone would want to copy a tubby underpowered thing that handles badly and blows its radar stealthiness by having to fly with the bomb bay doors open most of the time in order to avoid overheating.
The americans didn't want the MiG-31 to build their own one, they wanted to find out what made it tick and how to work around it. The factor of using vacuum tubes was mindblowing to them and made them think about emp resistance in their own aircraft.
That's _why_ everybody spies on everybody else (and if you think 'allies' aren't as busy pilfering data about each others' systems or industrial processes, you're pretty slow)