I agree with some of your thoughts, but disagree with others.
For a start, the bike above has disc brakes so has got some clearance for the mudguards, but I'm wondering how long before the mudguards are packed up in some situations? You're lucky there because I can see a good 1" or more of clearance, this would be impossible if you had v-brakes.
I'm not lucky,I simply chose a machine capable of fitting m/guards to and still having huge clearances.The discs(or drums) are a must for such a bike.
Which brings to the point of why mudguards went out of fashion for recreational use.
A hard use of the mountain bike would make the guards to rattle all the time and most likely crack. So, again, it's all about the user and use is made for
You're simply wrong in that assumption,the m/guards on that bike are solid and never rattle,they've been fitted so they don't,and will and do stand up to any kind of off road abuse proven many times on some of the roughest of trails.
As far as health issues, I always pack a waterproof or at least a windproof jacket and never get that cold. Always check the weather forecast beforehand!
I won't expect a mudguard to keep me warm in a damp day, just do what grandpa did and use that wonderful material that is wool for some of your clothes
I almost always carry waterproofs especially in winter,I'm not daft
That said I see a lot people on the moors who are quite stupid,but m/guards would go a long way to keeping them dry at least,providing it doesn't rain.
Wool doesn't suit everyone,I run on the warm side and find wool too warm for me. I just become saturated with sweat whereas the right synthetics work fine keeping me dry and warm.