As long as you treat the frame with Framesaver, rust really isn't a problem. There are tons of old steel bikes on the road, and they're holding up fine. And frame tubing is thick, so even if you do get a bit of rust, it isn't going to rust through like the body of a car. Plus, chromoly rusts slowly once a patina is on the surface.

I would be more worried about aluminum frame fatigue life.

All frame materials have their strengths and weaknesses. I prefer steel. But I have an aluminum bike too.