Using .PSD as the native format would be risky, its direction is set entirely by Adobe, there are dozens of different versions, and many Photoshop features which would be quite hard to support. And maybe some existing GIMP features would be hard in PSD too. For instance the non-destructive features in PS are mostly "adjustment layers", which might not be exactly the UI concept that GIMP wants to use for non-destructive editing.
But, GIMP does have a PSD file loader/saver. It is always in need of improvements, so any contributions would be very welcomed.
Then leave the legacy behavior in place. Telling your existing users that they aren't important enough compared to mythical new users is pretty poor behavior.
Never change anything, in other words.
Hobbling the save interface only serves to make the lighter weight workflows more difficult (because they're not glamorous for the GIMP devs) while doing zero to improve the more dev-favored workflows.
We do. All the time.
> For all the bleating about professional features, they sure miss big ones like CMYK support.
Which is being worked on.