I suspect the problem with M&S is that they have someone working high up in womenswear who had a very fixed idea of what different aged people should be wearing, and thinks that only the elderly would buy clothing from them.
I bought a lot of per una about 8-10 years ago. I like skirts and pretty blouses with a cardigan type of thing and I could walk into a per una section and find a dozen things. I got chatting with a lady (in her 80s, I was in my 30s) who was in there shopping and she commented that she'd been desperate for M&S to have a range like it.
I think M&S noticed that people over whatever age it is that they thought per una was for, were shopping in that section so they fixed it for the older people ignoring the whole floor of coloured polyester hell that they had already developed.
I've been keeping an eye on the Indigo range. Those M&S shoppers that persevered wandered over there to find attractive clothing. Someone from M&S must have spotted older shoppers because the range is going the same way (it was actually my mother that muttered about this).
They have something that is working, and then break it...