> I don't understand why silk would have durability problems.
Good question. Remember that the silkworm only has to make something which will last until the Pupae hatch out - a few months. Wool has to last the sheep for several years, but no longer. Natural fibres do bio-degrade. Synthetics are engineered to last a LOT longer, and don't bio-degrade.
I think another part of the problem is that 8 mm silk is rather light. Even though we treat our silk liners carefully, they do start to rip after a few years, usually at the seam near the top end. I suspect that your briefs are made of a much heavier fabric than this. Heavier => stronger. And maybe your briefs are not subject to the same tension as you see in a sleeping bag wall when you thrash around at night.
> Green Design
Sorry to be so cynical, but in my view most of this is just a marketing ploy. I spent 27 years in textile research in Australia with wool, and there is no way I would use it for any extreme sport. A LOT of R&D money was spent trying to get it to match the performance of good synthetics, with zero success among athletes who field-tested the ideas. I've tried it extensively in the field too, in the past.
For fashion and comfort at home cotton, wool and silk are great, but compared to modern synthetics they are weak (especially when wet) and wool takes for ever to dry.
I know I cop a bit of flack over my avatar :-) Those pants are Lycra ski-racing pants. I have a matching Lycra ski top. I was wearing them while out ski-touring, and it started to storm - with rain. I got wet. That is not a good idea in the snow! We kept going in the storm - partly to keep warm. When the rain stopped my Lycra started to dry out. It was effectively dry in 15 minutes.
Bottom line: there are many differences between natural fibres and synthetics, and performance athletes don't use natural fibres in their clothing - except maybe thick wool socks where the felting action is useful. But many do wear wool clothing at home or on parade.
However, this is just my experience and opinion, and everyone is free to make their own judgements about what suits them. That's what the Forums chat is about: sharing experiences.