Earlier quoted context omitted.
They could remove it in a new version of windows, that crusty POS system is going to be doing a lot of testing before they upgrade the version of windows it's running on. Even more likely is that they'll never upgrade from whatever ancient version they're using so it won't affect anyone. I think they just keep it because it feeds into the backwards compatibility myth that MS has built up.
Or they could just create a registry flag that enables the backwards compatible behavior. Then all default installs don't have the "bug", while those that need it can enable it.
Without a manifest, you'll see pre-XP common controls, and GetVersion will return an old version, so every modern application has it anyway.