Photoshop is an example of software that must also remain backward compat with the human systems that use it. There are lots of professions that are essentially "Photoshop Expert". If features are removed, altered, or even if keyboard shortcuts are changes, it has a very real effect on these peoples' livelihoods. There are few novice Photoshop users compared to other software.
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.
