Which order do you prefer them in? Since CLAUDE.md is no longer necessary, but was for so long, what happens when users only update AGENTS.md going forward while keeping a stale, unchanged CLAUDE.md around?
We're talking about changing default behaviors here in ways that can be surprising to users. It's reasonable to try and accommodate existing setups, future setups, and - yes - careless users.
At some point, the catering is not worth the sacrifices and future gotchas you're going to embed in the code.
There's no surprising behaviors in that situation. Someone who is aware of the change would most likely delete CLAUDE.md. Someone who is unaware of the change would assume CLAUDE.md is still the one being read by CC, which would hold true.