Moondog55 wrote: sleep system is that it should be adaptable to cope with almost any temperature on the planet.
It can work that way but there is wastage in weight and pack volume having to combine two sleeping bags or quilt with weight in the outer fabric of the secondary system. Apart from some flexibility in ranging the temperatures like adding a warmer liner and clothing, it’s better to have two specialised separate systems for extremes in temperature because even “almost every situation on the planet” varies too much -20 C sub arctic (more likely trekking winter in Nepal at 4000-5000m) to 20 C Mediterranean Sea level summer.
It also depends greatly on whether one is a warm sleeper vs a cold one varying by as much on average 5-10 C (some people way outside this range and beyond salvation needing more extreme measures).
As a warm sleeper my -2 C quilt with a silk or polystretch liner, t shirt and undies (warmer) or long thermals (colder) will do me for -6 to -10 C or my - 6 C bag plus liners down to -12 C.
The issues with oversizing a quilt or bag is you have to live with that more of the time when one doesn’t need to in terms of weight, bulk and cold spots (i.e why do mummy tapered designs bags exist) that the body has to heat extremity voids - many compromises.
It depends we’re one uses it of course. I have got by on a 0 C bag with liner for the last 30 years, on and off backpacking overland and many treks in Europe, Nepal, Africa, South America, Aust, NZ, Japan winter and summer. During that time my -9 C bag sat idle most of the time being too warm for 90% of those locations. One can easily dial temperature up but not so easy downwards unless just spread across the toes. That type of use travel, trekking and home hikes needs to cover use in hostels, trains, friends places, car camping, hikes, Himalayan treks, Mediterranean and tropics with night temperature ranges of only -5 C to 15 C so a sleep system in the lower middle temperature range of that should suit most (0 C for warm sleeper or -5 C cold sleeper).