Those Pentax pancake lenses go on the K-mount which is about 45mm deep, so the total camera depth is not so compact. It is the opposite extreme to Sony NEX where the mount is shallow but the lenses are longer. Also, as has been said many times before, not all lens designs that work well with 35mm film also work well with 35mm format sensors, with the problems being towards the edges of the frame. As far as I know, none of those Pentax pancakes has been tested for edge performance on a full 36x24mm sensor.
Not surprisingly, propaganda emphasizes images of the Pentax lenses (but not the 35mm-film format bodies they go on) and the Sony bodies (but not the zoom lenses that will mostly be used on them). And indeed you are doing exactly this: selectively emphasizing the NEX body size and the Pentax pancake lens size!
But yes, if all you care about is a small number of mostly slow and wide focal lens choices, a newly developed compact 35mm-film format kit is technically possible, but probably not commercially viable. But if the lenses are slow, how much advantage is there to to the big, expensive sensor?
Both 40´s and the 70 have been tested for 35mm stuff, and do remember that the regular Pentax FA50 1.4 and Pentax 43 1.9 are petite [incidentally, the only pancake lens as such -tessar design- is the 40 2.8 Ltd]. Not the 21 that much.
Nevertheless, it is true that, in the end, there is a certain distance that must be kept between the image plane and the last glass element. Sony decided to put that distance within the lens body, while others have decided to put that distance within the body of the cameras.
However, there were, as well, many, many other types of cameras which can be applicable in this case: folding and collapsible point and shoots as the Espio line, or the Ricoh GR; collapsible lenses for storing, and the Konica Hexar lenses.
Although, the "low pricepoint" should be more or less "under 800 euros", and then we will start talking.
