For whatever it's worth, the animals are going to get killed either way. Absent the brush market, true and proper kolinsky sable is going to die either way. They're pests; a great deal of their fur goes towards coats or other clothing, and only a small portion of the animal goes to brushes; and as far as ethics go, the fact that they're wild-caught means that they live much more comfortably and pleasantly than most animals in agriculture in the west.
Like Nan said, synthetic brushes are petroleum products. They're all plastic. Completely ignoring quality (I can say that my experience with cheap says-sable-and-is-definitely-real-hair-but-probably-not-kolinsky-sable brushes has been better than synthetics, and i'm using the bare minimum), they just last a lot longer. With proper care and keeping I don't see why a high quality natural hair paintbrush couldn't last you well over a decade.
We're comparing two issues, basically: the use of petroleum products in brush production, which need to be replaced relatively frequently, shed microplastics in the water, etc. vs. sable hunting, which involves the death of an animal, but a single death can result in a set of brushes which will each last a very long time. Plastic use vs hunting. I can assure you that one of them poses a much greater risk to the environment; if you consider the ethics of the environment as important as the ethics of the creature, that is important to consider imo.