The thing is they have changed, but for the worse. My mum and Grandma used to buy clothes in M&S when I was a child (1980s) and the quality was lovely. Nicely cut, quality fabrics. You would buy a classic item and it would last for years. I can't imagine that much of their stuff these days would last half as long. Which means they alienate the customer who wants to buy something that will last. But the customer who is happy with more throwaway clothes wants something a bit more fashionable, so they don't shop in M&S either.
Their old business model might have been boring, but at least it worked.