Why hasn't Nikon released a f/2.8 DX zoom?

'Reach' through crop sensors is as you say only magnification. It works as long as the following are true:

• pixel densities of DX sensor is higher than FX sensor, and
  
• lenses are able to resolve to the higher level of the denser sensor

Cameras like the D800 already exceed the D300s in pixel density, and even the D600 is approaching it. The ability of lenses to resolve is not infinite either.

Higher resolution lenses cost much more. At some point, it will be cheaper to create a larger lens than an equivalent FOV higher resolution shorter lens. This is probably already the case with micro four thirds lenses. Precision manufacturing is expensive.

In any case, there are absolute limits on resolution dependent on aperture set by diffraction. For higher pixel densities you need higher resolution and wider aperture lenses - i.e. expensive.

For the reasons above, in the end the larger FX sensor will ultimately win out for reach.

People always cite the percentage of sales which are DX as evidence of its importance to a company. That's only part of the equation - more important is what are DX's contribution to profits?

DX looks largely like a very low margin highly competitive and price sensitive business. It is perfectly possible to be producing a huge number but not making any profit or even a loss.

This isn't about what companies should do or what I would like them to do but about what I think will happen, just as my previous post was about what I think they are doing.