I guess I agree and disagree Pete. Foil sleeping bags aren't a complete scam, they're just often put to inappropriate uses. For more than 20 years I couldn't leave on any dayhike without packing a foil blanket and Tacoma Mountain Rescue emergency tent, because basically that's what prepared hikers and climbers of my particular vintage would do. I agree, the typical foil blanket isn't good for much more than temporarily placing over a patient during treatment in an effort to block wind and retain body heat. A blanket is far less efficient in design than anything that wraps around your body, like a bivv sack. And a sleeping bag (or two) is always better, but who always has Better on them in an emergency. Foil emergency blankets and TMR tents were compact, and importantly would keep you dry in an emergency, which by itself ups your odds of survival greatly, at least here in the moist Pacific Northwest.

Times change, products change for the better. The AMK bivvy isn't foil, it doesn't rip like foil, it reflects heat, and does a decent job at that. I agree, a bivvy without some ground insulation below you in most terrains won't do you much good, but with an insulating pad (either body length or smaller for sitting up while trying to sleep) you will conserve body heat, and stay dry. And in a pinch, rip down some branches, make a pile, and make do with that for ground insulation. I have spent a comfortable night outdoors in an AMK bivvy, with full outdoors clothing. It acts as a wind break, and reflected heat as well. So my daypack these days includes ground insulation - I find that a Big Agnes Air Core mat is a good compromise between insulation and packable weight / space - and an AMK bivvy, because without one the other is pretty ineffective. Although, if I am dayhiking where I might see sudden snowfall, I automatically pack a real bivvy sack (like my Montbell) and sometimes a small down sleeping bag. The Montbell bivvy is light enough that I should really just pack it all the time, but the reality is I pack an AMK bivvy for rescue situations, where I may need to slit it down the side to get it around a victim and conserve their body heat. Sure, I would to that to a $99 Montbell bivvy if I really had to, but I'd rather do it to an $11 AMK bivvy instead.

I think it all comes down to what options you have, and what you can improvise.