I don'T think that the TOHKBD is to thin. The problem in my opinion is, that J1 with attached TOHKBD is horrible unbalanced.

And I want a mobile device not a brick. If I need an extra bag to take it with me I will always prefer my ultrabook with a proper OS and a lot of performance.

The J1 with attached TOHKBD is already a monster. despite the fact that it isn't much thicker than the n900. But the N900 is much smaller so the thickness isn't that bad. With a much bigger device the thickness has to be reduced otherwise there is no way to put it in the pocket of your jeans. 12 mm sounds reasonable for me. A 5.5" screen device with 18 mm isn't a device I would buy.