I think I would deal with this on a case-by-case basis. If the project was primarily for Objective-C/Apple I think I would use -Werror against the most recent set of tools. My experience writing software for the iOS platform indicates this wouldn't be an onerous initial constraint. And, there's nothing to stop others from changing the build settings.

I would absolutely relax the constraint if I was maintaining a large cross-platform OSS project. In that case, I think your criticism is on point. I hadn't considered this situation as I've not been the maintainer of such a system.