I have not had any recent trouble with M&S.
When my washing machine died in the summer, I did not need to buy emergency undies but used all of the 55 pairs I had hoarded over the years and eventually washed when the new machine arrived.
My skimpy M&S offerings, bought in the 1980s had mostly been resewn where the elastic parted company with the fabric. (I could sew then.)
The Woolworths knicks' elastic failed and were mostly thrown away when they didn't stay up.
The BHS Size 18s I bought in 1984 were still OK but some developed small holes. The labels were still legible.
Some of the larger M&S offerings from 2009 hardly stay up now.
My latex-allergic friend told me latex was replaced (with Lycra, I think) so that people would not suffer anaphylaxis from their undies. I'm not sure when this happened but think probably 1990s.
The knickers before this change have stood up well, unlike their wearer.
My post 2000 purchases have not.