I think the scope of your consideration is a bit narrow. Lockheed Martin probably isn't very interested in stealing technology from Sukhoi to clone and put in their own jets. However it seems quite likely that Lockheed has an interest in the particulars of Sukhoi jets, their weapon systems, radars and EW systems, and airframe capabilities, so that Lockheed may better design systems meant to counter those systems. Certainly the design of effective stealth aircraft required knowledge of opposition radar systems.

As for the American military being top dog by a long mile... that seems true, but you don't stay that way by being complacent.

You also don't ever stay on top by stealing/copying from others. By definition that always puts you (at least) 1 generation behind.

In simple scenarios with two actors perhaps, but I don't think that holds true for complex scenarios with multiple participants. Somebody who steals the best from everybody and is always one generation behind the state of the art in every regard may still prevail against all others because they are a Jack of all trades, master of none.

I'm not saying there isn't use for knowing your adversary's capabilities (especially in designing your own offense/defensive systems) but very little of that knowledge informs the core of future arms programs.

..unless you are China