Trying to reduce UI design to something like Photoshop seems quite problematic to me. It encourages the designer to think that what you see is all there is. What about the things that aren't immediately evident in a WYSIWYG environment, like accessibility for users with disabilities (I'm thinking especially of blind users)? And if the WYSIWYG environment uses absolute positioning, as Photoshop does, then how will the UI adjust itself to different screen sizes? An advantage of representing a UI in a human-editable format like HTML, XAML, or Android's XML layout system is that all the properties of the UI elements are equally visible and editable, not just the things that are visible in the on-screen rendering. Also, I think that reading and editing the UI definition in a textual format forces the designer to think of the UI at a more abstract level than just dragging and dropping elements onto the screen and tweaking the things that are visible in that environment. And for the UI definition to actually be human editable, one is forced to leave absolute layout to the computer, making it more flexible.
Photoshop isn't even used much for visual design these days: it's all pretty much Illustrator. Vectors make dealing with resolution much easier, but redlining is still needed when rasterizing to lower resolutions.
Most of the designers (outside of web to be sure, since I don't live in silicon valley) I know don't use textual formats, that is for developers. Much of the content is too dynamic to work with textually without substantial programming.
We're doing some clever stuff between constraint layout and relative layout that will make a lot of that concern disappear. We also intend to present this as almost a statemachine, or a storyboard where you see the various states of your UI laid out, which gives us an opportunity to surface a lot of the "unseen" aspects of UI.
