Leo,
As I recall, NS did a check of their first aluminum coal hoppers, and found that there was very minimal loss of thickness over the course of the cars' first 25 years in service. I don't know how much it costs to patch up a steel car with new sides/hoppers, etc. I know I've seen some of NS' cars used in Central App service and some had at least two newer layers per side. I would have thought that the additional costs for re-siding the cars, along with the extra fuel costs to move these cars vs. aluminum cars (after all, a reworked steel hopper could weigh 60k+ pounds compared to 46-50k pounds for an aluminum car) might convince them that aluminum was the way to go. But the difference in initial costs plus the fact that you might actually be able to repair a steel car that's been in a derailment, probably make sense to a railroad, where it doesn't make sense to a shipper like my company.