It is easy to reason about the performance issues of CoW, and it's easy enough to reason about how to work around those issues. Ease of reasoning is a big deal

It is easy to reason about the performance issues of CoW, and it's easy enough to reason about how to work around those issues. Ease of reasoning is a big deal when you're dealing with hundreds of millions of lines of code, as some of us do. Cognitive load is a big deal in today's world. It's why we're migrating from C to memory-safe languages.