One of the first survival techniques I learned was how to rig up a one-man emergency warming station using a large garbage bag and a candle or, ideally, a carbide lamp. Essentially you cut or tear a tight head hole and slip it on, pull it down around you and sitting cross-legged you light your candle and place it between your legs. Even a fire as small as a candle will produce a lot of heat

It was taught primarily to river rafters and spelunkers, people who are likely to find themselves cold and short on equipment if things go wrong in a big way, but it works pretty well for anyone who finds themselves cold and/or wet. Condensation is not an issue because ventilation is simple, intuitively obviously really, lift the hem and fluff out the head hole.

Yes, being properly equipped would be better. A bombproof tent, down sleeping bag, full set of base, insulation, and breathable shell would all be very good. For even more safety and comfort you might stay home. Central heating, a down duvet, fuzzy pajamas, your latest Netflix selection, and delivery Chinese food would make for a much safer, and much more comfortable, evening.

Come the day when you find yourself cold and wet, and short of equipment your going to thank your lucky stars if you find a garbage bag in a hidden pocket. You can't let the best be the enemy of the good.

I don't see any great danger in people storming the great Northwest after giving up their regular gear in favor of a couple of garbage bags. That said, in addition to the odd mini-Bic or matches and tinder in foil tape, space blanket and small hanks of cord I stuff the occasional garbage bag in day packs, and jackets I'm likely to be wearing outdoors.