No, it wouldn't cut backwards compatibility at all. They have made plenty of additions to the format over the years, and they give you the option of saving to be compatible with previous versions. They could just keep doing that.
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.
