Even "down" cushions in high-end couches today are usually a PU foam core wrapped in down batting. It's impractical, expensive and unnecessary to do otherwise.
And "down" pillows are often 95% feathers 5% down, unless advertised as 100% down (and will hundreds more)
I have to disagree, PU foam has been the most useless cushion padding I've ever experienced in my life. It goes from firm + comfortable to useless in under a year for me in most cases. I've even tried replacing with the firmest PU foam available, and those attempts last maybe 2 years.
So I deem it as all those words you use, impractical, expensive, unnecessary to use PU.
An upholster can also usually wash and reuse horsehair when reupholstering a piece of furniture upholstered with horsehair. It's very resilient stuff(ing). You can also get horsehair fabric for upholstery purposes; it apparently wears like cast iron.
Am not an upholsterer, but I work with one somewhat regularly when a client wants an upholstered piece.
And "down" pillows are often 95% feathers 5% down, unless advertised as 100% down (and will hundreds more)
Not that down is wool, obviously.