The sort of bike buyer found on here is not typical of the cycling public. Most punters just want something affordable and functional to get around on. For them, budget 700c hybrids equipped with relatively cheap alloy wheels, basic rim brakes, and a quill stem are not seen in any way as negative features. They do the job and they don't break the bank, and there are probably ten or more such cheap leisure/utility bikes purchased for every fancy road bike that might appeal to a "serious" cyclist.. Discs are certainly becoming more common a sight, but they are not going to displace rim brakes amongst those customers who see no benefit to them and are not prepared to pay a price premium for discs. You only have to look at how 6-speed freewheels are still commonly fitted to lower-budget bikes to see that not every cyclist wants or is willing to pay for extra gear ratios or any other supposed "upgrade".