Samsung was using premium plastic for its flagship smartphones the longest of all manufacturers, but eventually they also switched to metal design. The same thing will Microsoft do, as the company plans to use all-metal design in the upcoming Lumia flagship smartphone.
Don't expect to see the flagship device from Microsoft before Windows 10 Mobile becomes available. All Windows Phone and Lumia fans are definitely eager to see this kind of smartphone, especially since there are no real Windows Phone flagship devices around.
What the real question is if Windows 10 Mobile can save Microsoft and successfully compete with Android and iOS? Personally, I think it will take one or two Windows iteration before it becomes competitive.