That's utterly silly. If you take yourself seriously as a graphics editing program, then inevitably you have to move to one format that you can then convert into others. Otherwise you have to implement every single operation on every single file format you support. With the One True Format, (.otf) you can convert your .jpgs to the .otf and then do the operation there.
This then presents a problem. Tech-savvy users know that there will be a loss of data when you save to a consumption format. But the rest of us don't. If you allowed open and save in the same format, people will make the intuitive leap, that they're actually editing the files in that format, when they're not. They'll think all the layers they made will carry over into the PNG and that they'll be there when they re-open the file, when in fact they won't.
It would be a nightmare, because nobody will know what will save and what won't. You'd have to then educate users that to get everything to save properly, you have to save to .otf. Or you could just bake that logic directly into the app.