Hi Space The reason why man made fibres are incorporated into natural cloth is because they can help minimise some negative characteristics of a natural fibre. A sweater made of 100% cotton can be very heavy, so heavy that it drags itself out of shape whereas if you add a man made fibre, it holds its shape better.
The days when polyester was the cheap option are over. In fact when it first came out it wasn't seen as a cheap option at all. Many of these fabrics are chosen because they are actually very durable, take dye in a particular manner and hold onto it or because they wash and iron or dry clean better.
A lot of cashmere and wool items pill, even the higher end stuff can do this. They use very short fibres (sweepings) which will not last. Cost is not always a reliable indicator unless you buy Lora Piana or Brora.
Where I see problems is in the use of techno fabrics- wool bonded neoprene, raffia bonded onto leather, metalwork, embroidered leather, plastics, feathers and mother of pearl paillettes. How in God's name do you wash or even dry clean some of these clothes? It can cost over £700 to dry clean a high end item.
And I am sick of buttons being so badly sewn onto costly garments. Plum Sykes wrote a great US Vogue piece on this some years ago after the buttons fell off her new Stella and Balenciaga coats.