The Americans have more to lose in such a trade. The F-35 is more important to the US and nations using it than the S-400 is to Russia. In any case, SAMs are very specific in their role which is contrast to a multirole fighter/attacker. SAMs are also at a huge disadvantage to begin with in a SEAD vs SAM fight. A well equipped force of 4th gen fighters can easily wipe out S-400s. Why? Because there are very limited number of missiles they can launch to intercept whereas a fighter can go in and out of it's range as long as it avoids the NEZ... even then, there are numerous ways a fighter can challenge an NEZ. So many decent anti-SAM weapons are available already. If such a "force field" SAM is ever made, it will still run out of missiles whereas the fighters can re-equip.And yet it never cross American minds that they can also study the electronic signature of the S-400 in turn. The most premier Russian SAM atm.
No doubt air defence has a worthy place but we're comparing apples and oranges.
The initiative is entirely on the side conducting SEAD. So for such a trade, the Americans will be getting less by knowing all the "super secrets" of S-400 whereas the Russians would be gaining a genuinely valuable insight into F-35's radar, heat, electronic signatures.