Air is not a good insulator when compared to water. Look at coolers.
The wall of a cooler uses air for insulation. You're confusing insulation and thermal mass. Filling a cooler with water increases thermal mass, but the insulation comes from the air.
You're example is flawed as it has an insulated bag. That's the key to your idea, not the air.
The bag insulates because its filling contains air. Fillings like hollowfibre trap a lot of air and prevent it convecting, so there's very little heat transfer. That's how sleeping bags keep you warm and it's also how the wall of a fridge stops heat getting in. The air inside the bag isn't the main form of insulation. You want the air inside the bag to circulate and conduct so that the heat energy in the beer is absorbed by the ice bottle, which will lower the beer temperature.
And I still don't understand how your magic sleeping bag only takes a bottle a week when my chest freezer runs multiple times a day, even with a thermowell measuring the exact temp of the beer.
I use one or two bottles to get the wort down to pitching temp and then change maybe once or twice in the week, so it's not just one bottle for a week. As I said earlier, I've only had to change the bottle once in the last week.
Lowering wort 20° with either method isn't practical.
It is with adequate insulation. A two litre (about 2 quarts) bottle ice from a domestic freezers is a lot colder than freezing temp. They come out of my freezer at -27 Celsius but an average freezer will get ice to -18 Celsius. With perfect insulation that can lower a 5 gallon (19 litre) batch of beer to 7 Celsius (45 Fahrenheit).
http://www.onlineconversion.com/mixing_water.htmIn practice insulation isn't perfect and there's some heat energy from the air inside the bag and the bag itself, so the beer doesn't get quite that cold. Fermentation also creates a bit of heat.
As I said, I've tried both methods. If you've only tried one method and not the other, perhaps give it a go before offering an opinion on whether it works.