Is a 9-18mm Mark II inevitable?
Personally I think the build quality of the Oly primes is just fine. My metal Nikkors were/are more robust, but I've had plastic lenses for nine years that are still just fine.
But the 9-18mm... The collapsible design is so neat for portability, but I do worry a bit abouomitted when the lens is extended. It just seems more vulnerable than most to dust and moisture. But if you get rid of the collapsible design, which is probably necessary if you want it weather sealed, then it becomes a different lens. Perhaps they should offer a 9-18mm MKII at a lower price point, and then a larger weather sealed 9-18mm f/4 or wider.