No idea. I've only changed brushes in steering pumps for ships. I've encountered pumps older than myself, and they still work fine as long as the brushes are changed from time to time.

If the brushes are expected to last longer than another part whose failure makes the unit a total loss then what do they care?

But yeah that's going to be noisy both in terms of audible noise and electrical. We have much better options these days and they aren't even that expensive.

Older front loaders i think. The speed and direction control is much simpler compared to modern BLDC.

I don't know about "most" but Universal motors use brushes and can run on AC.

Value engineering: engineering the value right out of the product and into the shareholder pockets

Probably. In Swedish many forms of Carbon share the same word "Kol". It is the word for the element Carbon, but also the fuel Coal. Carbon brushes are called kolborstar "Carbon/Coal brushes".