I was initially sad to see this happen. But reflecting upon what's at hand, this is the logical thing to do. They're doing to photos what they did to video with FCPX. Except, they wanted to avoid the FCP X debacle about releasing half-baked "pro" editor when they had to rewrite the code from the ground up.
There will be some growing pains for sure, but in the long term, it will be good. There will be plenty of inter-platform compatibility, something that's not present at the moment because of the original programs were written before cross-platform is viable.
The API extensions will be a boon for developers as well. it will spur competition and creativity. Al this extensions will live inside Photo app. So I'm hopeful