Let's face it, most cars travel a big portion of their miles with 1 person in them. A sensible rack and some panniers and you can carry most of what you need. How much does a child carrier to go on the rack cost (when they are too big, they can ride their own)? That's the majority of utility cycling. That also seems to be what you see in pictures of Copenhagen or Amsterdam or wherever.
I think that when you start talking about esoteric cycles that allow you to do stuff that normally you can't do on a bike, you end up putting people off. So the electric bikes that mean you can commute 20 miles each way, or the bakfiets that means you can carry 3 kids or really big objects, or whatever other circumstances you want (tandems?) - these are specialist requirements needing specialist vehicles. Most people don't need one, don't have anywhere to put one, and wouldn't use it if it were free.
IMO, facilities that make utility cycling easy and low risk are the beginning, middle, and end of utility cycling. As the superhighways show - build it and they will come!