Galvanic protection prevents rebar rusting. In places like the Bay Area, if you look around major concrete structures, you'll see "anodic protection" plates on the ground which contain large blocks of zinc. As long as you keep that zinc block fresh and the circuit intact, rebar will never rust.
Some protection also uses electrical power. I seem to recall there's a bridge somewhere that uses solar power to keep its rebar from rusting.
It doesn't have to; the deeper the rebar the longer it will last. Rust takes more space than iron, so it can't form if the integrity of the concrete is uncompromised. You have to weigh the extra concrete against the likelihood the structure will be obsolete anyway. Building a 4-lane floating bridge across Lake Washington that drains runoff directly to the lake wouldn't have mattered if it would have lasted longer because it was too small and environmentally damaging in other ways.
