So just out of curiosity, why doesn't Adobe embark on those two projects? We're entering a new era of desktop software, surely that should be a catalyst for redesigning both from scratch?
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.
