Is obvious to anyone that the problem here would be substantially of economical nature; both systems in facts are by now effectively frozen for what concern their inherent design structure and overall capabilities, therefore the problems that would results in the operational service, in same area, of both systems would be in the export potential of those systems,now high in demand world-wide (the F-35 program rather enjoy a "participation" basis even stronger due to the huge pressure exerted by USA on the nations in its sphere of influence).
If the F-35, in facts, would result detectable and trackable only at few dozen km by С-400 's multi-band complement of sensors, as portrayed in all those years by the most irreducible advocates of this colossal aerospace program (the most costly and lucrative military program in the history), export not only of С-400 but virtually of all other domestic air defense systems would suffer a true deadly blow and the same image of Federation's Defense Industry and aerospace security, now already centered around S-400 divisions will be utterly tarnished.
The thesis about "stealth" technology inalienable centrality and superiority in comparison with the most modern air defense technology will be fully proved.
On the other end, if С-400 battallions would be capable, with those same suit of passive and multi-band active sensors, to detect and track also in the most challenging jamming/decoy environment those F-35 at practically the same range of any other aerodynamic target, as portrayed by domestic developers, will be the US's military industry and nation security at suffer the same enormous backlash and horrible image's damages previously described for domestic side; with any potential customer of F-35A/B/C -aircraft putting all theirs eggs on the "low observability" and sensor data integration on a very low-performance airframe with strong limits to the kind and quality of weapons systems internally carried- not obliged by already signed contracts very likely to abandon the procurement at instant, very likely just toward domestic or Chinese products conceived around completely different design philosophy and interaction with advanced enemy air defense systems.
Obviously in this instance the ocean of words and conjectures ,uttered in all those years, about the mythical possibilities offered by supposedly "VLO" aircraft against advanced enemies will lose suddenly any meaning and theirs advocates will be pointed up for decades as example of self-delusional arrogant fouls.
At the moment it appear that "Алмаз-Антей" do not see any problem in putting an export version of S-400 operative in the same area with this US-made supposedly almost-invisible wonder Aircraft,protected by a true sphere of "situational awareness TM" , in this way potentially ruining completely not only its export potential of any kind of modern air defense systems around the world, now and in the future ,but also any chance of procurement by part of Federation's MoD for aerospace defense, neither any domestic General or political figure has ever attempted to prevent S-400 to be placed in the same area of the US made wonder aircraft.
On the other side you observe the exact opposite behaviour.............anyone can suppose WHY..........
Gen. Mike Holmes recently anyway has also strongly advocated for collaboration with US and Allied Armies in the efforts for defeating С-400, obviously domestic officials have already responded to this laughably simplicistic approach to the question of confronting with all theirs forces only С-400 divisions ;pointing out that not only any other class of air defense systems but as also domestic space based assets, front and long range Aviation, Army offensive and defensive systems ,Navy long range missiles ,EW and masking equipment, VDV Airborne Divisions etc...would be all devoted to destroy theirs Aircraft, dismantle theirs airbases, radars and command centers.
If to confront only С-400 battallions western Air Forces generals need the collaboration and involvement of so much assets and resources of theirs armed forces they are truly in a sea of troubles ,first of all because at the moment i fail to remember where are theirs remotely distant equivalents not only of С-400 divisions but also of Панцирь-С/C2/CM ,Тор-М1-2У/М2, Бук-М2/M3, Верба, Леер-2/3, Красуха-2/-4, Мурманск-БН ,ТДА-3 etc...preventing theirs command centers, radar installations, and air bases to be reduced in few minutes to big scrap landmills.