Teenagers, or some of them, would use it until it walks off (see my earlier post regarding a not-teenager and his duvet).
Duvet covers are not difficult to change although the job isn't something I look forward to, however when I do it, it's soon done and I think, "What's the problem?".
I saw the under blanket but can't really see the point. Sorry. It would need to be cleaned fairly regularly too and dry cleaning must have some sort of non-green, environmental impact. However we all have stuff dry cleaned sometimes and this is no criticism Rose, if you find the blankets good that's great. I imagine such blankets will be fashionable for a while.
Wool of all types makes me itch, a lot of people have that problem. You can buy wool that has been treated, including cashmere. 'Cashmilon' is a very good alternative to wool and doesn't look cheap - but that's clothes.