Erosion has more than four states, though it still only displays as “eroded”, “very eroded”, or “thoroughly eroded”; different rusting sources can cause different levels of rust (e.g. dipping a long sword in a fountain or getting hit by a rust trap would probably only cause a small amount of rust, whereas a rust monster might cause a lot.) This would enable a more fair way to have things “erode away completely” like they do in Grunthack, since you would be able to disengage any thoroughly eroded piece of gear before it was at serious risk of vanishing.