Seconding @leenerella's comment. While I can only second-guess how genetics, hormones, and follicles will play out for you, your arm hair may in fact be getting lighter, but you can't see it right now.

One thing I did was to invest in a hair trimmer that I could use to shorten my armhair, which would let you keep your arm hair but gradually trim away the darker and coarser parts of the hair. What grows in its place is often less dark and coarse. And since there was enough hair grown out, it didn't feel stubbly. But YMMV--I never had heavy body hair.