Of course, optimal companies maximize for margin. Buyers have their own optimization mental model and maybe is surprising but a vast majority are thinking mostly on cost. Buyers and sellers do a dance and in the perfect long run you hit the optimal balance.

Consumers/buyers still play a large role in this, it is easier to put all the blame on PE or Big business.

This is the conventional thinking, but it ignores a huge factor - marketing. The major function of the gigantic advertising industry is to deceive consumers about the real qualities of a product, leaving price as the only only signal that they can detect through the noise, in most cases.

And advertising works in multiple ways to promote slop. Sometimes, it is directly by marketing bad products as cheap but high quality bargains. Sometimes it is, as I said, by using previous high quality products to sell low quality ones at the same prices. Sometimes it works by creating a huge pressure to consume more (such as the pressure on fashion trends), wiping out any care about durability (if it's considered poor taste to wear the same T-shirt two seasons in a row, why pwuld you buy a durable T-shirt?). Sometimes it works by mudding the waters, making consumers distrust any reviews that praise the quality of a product, leaving price and directly visible looks as the only signals that rational consumers can base their decision on.

So, overall, the blame for this state of affairs lies far more with the way the modern market was designed, than with consumers specifically.