With the proliferation and reduced cost of black market/ gray market MANPADS, I'm not sure where "permissive airspace" exists. Syria sure hasn't been permissive for Russian fixed and rotary wing aircraft. Mogadishu in '93 wasn't permissive for CW3 Michael Durant.
We ain't the Russians and we aren't flying Blackhawks 150ft over downtown, the way we operate makes it permissive to us.