Hmmm... one thing I've noted, is that nobody has come up with a serious rebuttal to my terrorist miss-us of GPS scenario
You've rebutted it yourself. You are saying that shutting off GPS will cause havok. That's precisely the argument that others are using to say that it won't be shut off...(even in the event of terrorist use) because it would cause havok, and not only to aviation.
You have raised a valid point, but the risk is low and the
safety consequences to aircraft are not major.
Using Dick's affordable safety argument, it appears that the cost benefit of shutting off secondary radar and using ADSB is compelling. It's already being used in areas that previously had no radar like coverage.
To answer your question,
if the GPS signal is interupted,(which is unlikely), then we will revert to procedural separation where ADSB is being used to separate. Aircraft will be delayed but safety will not be compromised. There is certainly no doomsday argument to be made here.
I am not in favour or against replacing secondary radar with ADSB, but I don't find your argument a compelling reason to stop ADSB introduction.