Ian W :
So tell me again why it was a good idea to go to only cooperative surveillance systems?
Only money ( it usually is ) a Primary Radar is extremely expensive compared to an SSR one , and you need a mode S radar SSR anyway if you want to implement the future tools, so PRI becomes an expensive back up..
a 9/11 issue where the bad guys know to turn off the transponder
No, the military still have primary,(and other means ) to plot aircraft , it will only affect Civil ATC, Most Air defense systems track aircraft irrespective of Transponders.
you can get the same effect from a lightning strike
Maybe in the past , never seen a total electric failure in a civil a/c from lightning in decades. But total loss of transponders due human error, oh yes..
Incidentally I was told this is not 2 cases of B787 total SSR loss in the last 6 months, , but 3, and with 3 different airframes. (AI had 2 of them )