> The point is precisely that we don't change anything: applications
> checking against sys.platform are already broken, there's no reason to
> comfort them into using this defective check.

You grossly misunderstand the concept of "backwards compatibility".
At times, features get added to allow even buggy (or perceived-buggy)
applications to continue to work under a change.

> The applications that encountered the problem (chromium, matplotlib
> and probably others) already performed the change to
> sys.platform.startswith(), so it's really the only way to go.

I'm very certain that not all applications have been changed yet.