There has got to be an option for a fleece-like material that doesn't involve significant pollution or significant animal suffering. At some point, fleece has just gotta be phased out if we can't produce it without enormous downsides.
I wonder if it’s possible to make fleece from hemp.
plant fibers can't replicate this.
polymer fibers can replicate this because they are solid extrusions.
The fibers are stronger than polyester, so if you get a thick silk fabric, it will last a lifetime. Unfortunately, it's absurdly expensive, so even though the total cost of ownership isn't terrible if you get a thick fabric because it will last forever, it's a ton of money up front. Additionally, most places sell super thin silk to keep the price down, which means it doesn't last, so you have to shop around and find the good thick stuff.
I love silkliving.com's 100% silk line, which even includes a fleece hoodie! I'm not associated with them other than as a satisfied customer that paid full price.
Apple stopped using leather and was criticised for environmental damage at least in the short-to-medium term (underutilisation of existing materials, new demand for plastics and synthetics).