Earlier quoted context omitted.
> They could just keep doing that Then it wouldn't be a new, "clean" format. Then it'd be the old format with additions. Which it already is. Your post makes no sense to me.
There is no technical reason they would need to use the old fileformat at all. "Keep doing that" referred to give the user an option of what photoshop version he would like the saved file be comparible with. If he pick the latest version, the new file format would be used.
so, .psdx