Dear,
I don't think US will ever put in Kill-Switches in FMS equipment. Think over this carefully and you will understand that Pakistan might lose a lot in that scenario, but US will turn out to be the biggest loser since the biggest export products of USA has been its military equipment for the last 2~3 decades.
If word got out that US made equipment was useless in war-time scenarios, more then a trillion dollar worth of military sales will be effected over a period of less than a decade. Do remember that only F35 foreign sales are proposed to be around 200+ billion dollars from 2015~2025, if we count in F16s, F15s, F18s, AH-1s, AH-64s, AWACs, their munition and supported equipment, Navy ships, Radars, Missile, Air defense equipment and Armored units, it will create a frenzy in buyer's market which will divert all trillions of dollars worth of arms money from USA to Europe, Russia and China, meaning decline of US arms industry and growth in their rival's.
Do you think US will ever do that ? just to punish a puny little no good country like Pakistan just to favor another no good Russian ally (from their perspective).
Its in their interest to keep India-Pakistan arming themselves with US equipment, creating jobs for their citizens at the expense of starving millions of our own. Its simple economic facts rather than complex political bickering.
Besides, if kill switches were indeed installed in US FMS equipment, how come Iranian F14s, F5s and AH-1 Cobras are still flying ?
Regards,
Sapper