Lower bandwidth for who? When images are cached on other instances, it allows two things:

• Load sharing. The original instance doesn’t have to serve the whole fediverse, but only its own users + 1 request per other lemmy instance.
• Data availability through redundancy. If the original instance goes down, the cached image is still viewable on other instances.

I must say it was a funny rant to read (and I 100% agree with you)

Edit: though i quickly looked at the code and it seems that the regex is customizable? https://github.com/LemmyNet/lemmy/blob/e181f4f41d61832f1278775179b6743699f8af12/crates/api_crud/src/site/update.rs#L95

(Edit: updated the link as a github permalink)