I think the point is that most folks in the west (especially in the intermountain like Shaolin) don't consider 20F+ bags to be 'winter bags'. My lightest summer bag is an 800-fill 32F. On the other hand, I just saw a Marmot 0F/600-fill bag for $230 online (so it is possible if you want to go 3.75#)

I guess it all depends on the application and what you're willing to carry vs. what you're willing to spend. For car camping where weight doesn't matter, you can't beat a real tent with a flannel/canvas bag. If I'm packing it, I go for the lightest (smallest packing) bag I can afford with the required warmth. If you're going to wear a bunch of clothing at night, you can go with a warmer rating.

And btw, I'd be curious how a bivy does with condensation in the winter - I've never tried a bivy in extremely cold conditions.