Hopefully we won't have to use Flash to make panels I remember they were a big deal when they came to Fireworks. One issue I see with this is that Sketch currently doesn't have panels so they would have to change their own UI to accommodate this.
I don't have a big issue with inserting views into the view hierarchy. The problem is that the varied approaches are causing some plugins not to work together. It's on my todo list to look into compatibility issues with other plugins. I'll probably come back with some thoughts on how plugins can coexist in the UI.