Terrorism *isn't* an existential threat. The Soviet Union might have been during the cold war but not a bunch of fulminating islamists. They are just a pinprick and we (including politicians) ought to man up and deal with them as such. There will be casualties, but that's one of the prices you have to pay sometimes for living in the sort of free society we do live in. And it is pretty much free, despite government trying to nibble away at the edges.
I've just been reading a book about Operation Sealion (Seelöwe), the putative German invasion of England in WWII. The restrictions on individual liberty then were pretty much on the scale of a military dictatorship, and people put up with it because of the perceived threat.
The lesson is that restrictions should be consonant with the threat to be acceptable, and IMHO the threat is real, but overblown.