@Noir, I agree
Today phones are absolutely too slim and a hardware keyboard / sliding mechanism need some space.
...but again, today almost every phone's thickness goes near or below the usability margin - harder to ensure you will not drop the phone and some of them are easily bending.
There is no need to create paper-slim phones by getting closer and closer to the usability limit of the phone and technical limit of physics.
I think, for a well-equipped phone with correct slider mechanism, 14 mm sounds not a really high thickness...