I came across this today and provided for some good reading regarding f-stop.
I love Moose's articles. Walks you in circles, shows you everything, and yet you still want there to be more. :)
What I am proposing is a nikon product, so build quality will be within the Quality tolerances of the nikon family of products...
You missed the point above - to achieve what you are looking for, quality has to suffer. It is quality that determines price, not the F-stop.
I still think that there will be a market for a F3.5 DX equivalent
Your analysis is a good start. It does leave out/address two key things in your argument:
1) Why f3.5 - what is the real, user experienced advantage of f3.5 vs f4.
2) Existing market of available products.
You only compared Nikon lenses - not the market. Market viability includes all the market players and products, not just Nikon.
Here is a quick list (Amazon Prices) of Tamron and Sigma lenses to Plug into your numbers. I willing to bet the gaps are filled quite nicely.
Tamron 17-50mm 2.8 = $440
Tamron 17-50mm 2.8 VC = $600
Tamron 28-75mm f/2.8 = $450
Tamron 70-200mm f/2.8 = $700
Tamron AF 10-24mm f/3.5-4.5 = $460
Tamron AF 60mm f/2.0 = $472
Tamron AF 90mm f/2.8 = $450
Tamron AF 180mm f/3.5 = $680
Sigma 18-50mm f/2.8-4.5 OS = $199
Sigma 17-70mm f/2.8-4 OS = $450
Sigma 17-50mm f/2.8 OS = $630
Sigma 24-70mm f/2.8 = $900
Sigma 70-200mm f/2.8 = $870
Sigma 70-200mm f/2.8 OS = $1400
Sigma 85mm f/1.4 = $970
Sigma 50mm f/1.4 = $488
Sigma 30mm f/1.4 = $480
Sigma 50mm f/2.8 Macro = $350
Sigma 70mm F/2.8 Macro = $500
Sigma 10-20mm f/4-5.6 = $460
I love Nikon glass and will save up for it. That was not always the case though. We all logically know that sometimes people feel a sense of security of a product Name and choose it over others without ever looking what else is out there. We do it with cars, milk, candy, laundry soap, toilet paper, dish soap - I honestly do not know why I buy Ivory dish soap other than my parents did. I think we do that with lenses as well.
I have gone through my fair share of Sigma and Tamron lenses, owning the current or previous models of so many it makes my head spin.
I can say two things of both companies:
1) Optically their "better" glass is more than a cut above Nikon's Consumer level and is very, very good.
2) They fill in the gaps that Nikon & Canon have left open.
Many (including I) thrash them a fair amount on here. Honestly I beleive it shows each persons persnickety/perfectionist level on certain things rather than their overall quality. They are not the perfection of Nikon's Pro lenses - but very close and fill in the price gaps quite well.