So Ethics are different than environmental impact. Rocks never bio degrade they are fine being in landfill. Just because something doesn't biodegrade doesn't mean using them is unethical. But kill a living being when you don't need to is objectively unethical.
Responding to the philosophical questions regarding ethics:
Using your yardstick, Horsehair brushes are OK because the hair can be obtained without harming the horse.
I would also suggest that unless you also outlaw ham, bacon, and pork, then boar brushes are simply using product that would otherwise simply be wastefully destroyed. So again, not as unethical as some would claim.
Further, using more plastic than necessary and then land-filling or ocean-dumping can harm other creatures.
I'm not plumping for one over another, just pointing out that all of these brushes come with some sort of ethical issues if you look hard enough. There are no absolutes. It becomes a question of doing the least additional harm.