The black iron is only for the uprights. The part that sees soil is galvanized. It's similar to the volleyball net setup we had in my yard growing up. That galvanized pipe has lasted 10+ years directly in the soil, so I'm not worried about this stuff rusting away any time soon. Even if it does, another $5-10 and 5 minutes of time is no big thing to me.
, one of the reasons I went with the threaded pipe is because it's no big deal to add another 6' section next year. I think 9.5-10' is plenty for year one, and I think 16' is plenty from then on.
The cable is rated for over 100 pounds (tensile) which according to what I remember from engineering classes, is more than enough.