Copying is not theft under any mainstream definition of theft. The word "theft" is used instead of the phrase "unlawful copying" to create an emotional response. If the crime doesn't deprive the original owner of their access to their property, then it cannot be theft. You cannot get insurance to replace your copied property. Even the term "intellectual property" is meaningless since: property laws are irrelevant to "intellectual property"; the term "intellectual property" groups copyright, trademark, and patent law in the same category despite all of them existing for different purposes and working in different ways.
Property and theft are unrelated to copyright. Anyone who disagrees is arguing semantics.