I hope you recognize that your claims are deeply flawed and don't align with the reality that most middle-class grocery shoppers experience. Regardless of what you think makes "sense", major supermarkets throughout the country have rough assortment parity (with minor differences for regional consumer preferences). In some cases the long tail is actually longer in lower median income cities because labor is cheaper and stores are physically larger (cheaper real estate). That Publix where I shopped in Daytona Beach is enormous.
Instead of making things up you can just go look. Many supermarkets have online ordering now so you can see exactly what they stock at each local store.
Yes -- I know that my experiences don't align with most middle-class grocery shoppers: I worked in the food business for a decade where assortment was literally my job. There is a lot that casual shoppers like you don't notice! That's actually built in.
In any case, I love most of what you have written here.
Online ordering enables larger long tails. In which market do you suppose online ordering is more common, Daytona Beach or Mountain View?
If you were managing assortment at a Publix in Daytona Beach, how would you structure your long tail? Would you look to a Safeway in Mountain View as a model to follow?
Instead of making things up you can just go look. Many supermarkets have online ordering now so you can see exactly what they stock at each local store.