I think what YOU want (really), is a bulletproof 24/96 usb. I really don't know whether you want to get into the business of writing device drivers for windows. What's worse, you'll have to also spend time maintaining them. That's what'll be the expensive part.
Are you going to make a driver just for windows 7? What about vista? what about windows xp? 32 bit OS? 64 bit OS? What about windows 8? mac? Why not Linux?
At the very least, you'll have to spend time testing and verifying that it works on each one of these supported platforms. And bugs. What about bugs? Will you also have the infrastructure to get bug reports and crash dumps from folks to investigate crashes that will inevitably occur?
I honestly don't think you have the infrastructure to take this on.
Now, from a customer perspective:
The Mac folks seem to have a SPDIF out, so they're golden.
Let's look at the windows folks:
a) They have a PC (like I do) with both coax and optical out. We're set too, and couldn't care less about usb because like you said, optical is better.
b) They have a laptop/cheap pc without optical out. Dude, just suck it up. We're giving you 24/96 bulletproof. what more do you want? If you really can spend $20 an album to get higher res, just pony up the money to get a decent soundcard with optical out or a laptop with coax.
Seriously though. How much music is really available today, in higher res?
A better question to put to folks might be:
a) How much high res music do they have today?
b) How much high res do they see getting in the future. And the high res folks: since they're so obstinate about sound quality, will they use usb, or coax?