As you pointed out, the lens is not interchangeable so they had to come up with something better then F/4, i don't have a lens slower then F/2
F/2.8 and F/4.0 is twice the light, that's not an issue when you've got the sun on your side but that's not how i shoot. I need light i need a lens that will allow me to cover various conditions, that's why i only use fixed focus lenses. I'm often at iso 800 1600 3200 and that's often at F/1.4.
An example of how i shoot
1/80 sec F/1.4 ISO 1600
The foveon sensor does not have a good rep with noise
A fixed focal length lens at F/4 is going to have a negative impact on its sales. It's not a mainstream compact; it's going to appeal to certain kinds of shooters, street photographers/available light shooters.
I like what sigma is trying to do and it may push other makers to address what many people are screaming out for. But the sigma is not for me
And to think, i started this thread
Cheers
Sean