I agree, but I've run into many libertarians who argue vehemently against anything that is based on "an extended defense" idea. Admitting that "an extended defense" idea has merit is dangerous to many libertarians because it means going beyond a black-and-white view of their principles. It means that we have to draw lines between the black-and-white positions and argue for where the reasonable line should be drawn.
I'll give a few examples. Many people argue against drug legalization as "an extended defense" against someone frying his brain with LSD, PCP, or cocaine and driving his car into a tanker truck full of liquid propane. Many people argue for laws against hard-core pornography as "an extended defense" against rape or argue for laws against child pornography as "an extended defense" against exploitation of children. For that matter, gun-control proponents argue that they are only wanting "an extended defense" against violence. Anti-sodomy laws could have been considered "an extended defense" against AIDS even though the total severity of the health threat posed by some homosexual behavior was not known before the 80's.
I think you've made an important point, but I've found that many libertarians have problems with the whole idea of "an extended defense." To make matters more frustrating, many others have a problem with this idea when it goes against the position that they want to advocate but will use it when it is convenient.
WFTR
Bill
Good points you make, and I think we'd both agree that "extended defense" could get extended indefinitely all the way to total lockdown, both at the border and within.
Then we get into discussions as to what points defense switches to offense. No bright lines wherever we like, darnitall.