This is why the only Fifties I have are manual lenses that I use for low light or for "artsy bokeh" type shots. On crop sensor, 50mm is not "normal" or "everything" lens. The main advantages of 50mm lenses are that traditionally, they are cheap, bright, and have good IQ. So I would keep one on hand, but I don't use it much. I wouldn't invest a lot in a 50mm lens, unless I had a specific project that needed it.
I find the DA 40mm to have a much more crop-friendly FoV, even though its only f2.8. But its still not quite an "everything" lens.
But doesn't the Sigma make various Foveon cameras with different lenses? And besides, by buying a foveon sensor camera, you are already making many sacrifices (battery life, write speed, file size and format).. why not one more on focal length?