They already released 5.0, and didn't change anything. What is more likely to happen is the development of a Plasma Mobile QML UI built on top of Kontact and the new Akonadi that you can switch between, and that the desktop version might just use a reactive designed mobile UI as default with the current UI being available as a toggle option.
That's a pity. I didn't get KF5 one yet since it's still in experimental in Debian[1] and I assumed it's not usable enough. Settings UI in KMail really needs some serious streamlining.
