I'm not a framebuilder, but in nearly fifty years of riding bikes in the UK, which is not, let's face it, notable for its arid climate, I have never seen an ordinarily-maintained steel frame fail from rust. Of course it is possible - leave the bike outside in all weathers, make no attempt to clean off the salt in winter, damage it and do no repair/repainting - but in practice, if the bike is given even a modicum of care, it's a non-issue.