My parents (suburban middle class family) still use the service to this day.
How do they justify it? The milk is better quality then anything that's mass produced in the grocery store (and my parents still go to the grocery store regularly) and doesn't contain all of the growth hormones. I would argue it is very niche though.
Of course, a $3.00 delivery fee hardly seems extravagant compared to Prim charging ~33x the average cost of completing a load of laundry in one's home.
Any reason they don't just buy organic milk?
*Disclaimer: I've been working on the same model as Prim for a few months and will be launching in a major city in the next few months. The economics are there, even on a small scale. Scaling up is where things get really interesting.