This is all speculation, I agree. But we need to be cautious with how we use our “churn budget”, and a bad design choice right on the tail of recent issues might be too much for our end users.
I was assuming this might not work, because the build backend is run in a separate process, so the -W flag won’t get passed through (I’m less sure about the environment variable, is it one we let through to the child process?) But regardless, I’m not convinced that’s sufficient for the “I’ve reported the problem back to the dependency that’s triggering the warning, now I just want to get on with my life until they fix it” scenario.
Of course, if we’re only displaying warnings for top-level requirements, as I suggested above, that might not be an issue, as the user should be in a position to fix the problem themselves (unless they are doing something like installing a project from PyPI that doesn’t publish wheels).