Here are a few points which occurred to me while reading the 2006 Unconventional Sleep Systems Manifesto
1) Mummy bags - is anything really broken? I'm not religious about my RAB Quantum 200. It's just very comfortable to sleep in, and weighs precious little. Why RAB has added a zip and 200+ grams to the latest model is beyond my understanding.
2) Why not question layer 2, the sleeping pad? Let's face it, John Wayne never blew up a Thermarest in any of his Westerns. I was unaware of sleeping pads when I started backpacking in the 60s. In fact, backpacking wasn't even called backpacking in Britain then, so things have changed. But one thing is still true. You do not need a sleeping pad for sleeping on turf, particularly in summer. Eventually, in the 70s, I became aware of the Karrimat, but didn't buy one because no need was obvious to me, even though I regularly camped in winter. Things changed when I started sleeping in bothies, to get away from midges and storms. Bothy floors are hard, but that's not the problem. The real difficulty is caused by their flatness. My torso ends up suspended between my highly compressed shoulder and hip regions. I have slept comfortably on pebbles by the seaside. Hip and shoulder hollows in the gravel distribute the load-bearing chore along my whole torso. Compression is low and sleep is good. Natural hollows are readily available in tussocky grass and it usually possible to find a pair which fit me. Sadly, bothy floors prevent sharing of the load so now I have a range of sleeping pads. (Old Karrimats never die, unlike Thermarests and Ridgerests.) There are a few reasons for not bringing in vegetation to make a mattress, including damage to the environment and ticks.
In defence of layer 2, I have to say that a short, Prolite Thermarest seems very homely when I'm under a tarp and the whole world outside has turned to mud.
3) Finally, if you've brought the wrong sleeping bag, as I did one January on a Derbyshire moor, put on your waterproofs, over all of your insulating clothing. If it is really cold, your waterproofs will probably be clean and dry so will not damage the sleeping bag. For some reason, waterproofs tend to be quite warm. Clearly this option is unavailable to anyone with a poncho-tarp.