You mean the pads? Those definitely are - that's one of the items you can get from Herman Miller themselves in their online store (vinyl or leather), and there are third party options too.
Even if you mean the arm rest mechanism, that's replaceable as well. As are cables, the seat, lumbar support, gas cylinder, you name it.
I meant the pads. And it looks like you are right. Mine were vinyl, and started to crack after 2 years. I'll prob get the leather pad replacements. I love my Aereon, it is such a pleasant seat.
Even if you mean the arm rest mechanism, that's replaceable as well. As are cables, the seat, lumbar support, gas cylinder, you name it.