What an entertaining thread.
Taking a step back for a moment, I've never seen Carnival ships as rusty as they've been recently which indicates to me that there is an underlying issue. However, unlike some I do not believe it's due to poor maintenance.
For example, the "new" Carnival Sunrise which just left dry dock less than 1 year ago already has extensive rust, specifically concentrated along the waterline and lower hull sections. Even a few months after in mid-2019 I noticed she had several rust spots.
Several Carnival ships exhibit this exact rust pattern (including smaller spots on balconies) which is also suspect. Even the hulls seem to be "yellowing" where there are not clear indications of rust, further indicating a paint coating corruption issue.
Paint vendors often tout "new formulas", which can be easily created; it seems Carnival tried one and it's causing this.
Unfortunately, as others have correctly stated, due to regulations and the multi-step removal process rectifying this is not simple which is only compounding the issue.