What sort of competition is there between supermarkets in the US? Where I live, I have a choice of six large chains from which I can order online, and they all have large stores within a few miles of me if I wanted to shop in person.
2020s: Today, living in a suburb of Washington DC, I have: zero Groceries within walking distance; driving distance: Target Grocery is closest; next closest are several International Foods type places like Fresh World, Americana Grocery, Todos Supermarket; there are also: Wegmans, an Aldi, a Lidl, 2 or more Safeways within 10 miles of me, 2 Food Lions nearish me, and 10+ miles away is Giant Food. Oh, and the only place that has Pibb Xtra - Walmart grocery. This isn't counting the places you have to pay to be a member: Sam's Club, BJs, Costco... I think that's all of them close to me.
2010s: When I lived in Brooklyn NY, there were three larger sized supermarkets (larger meaning larger than a hole in the wall) within 5 to 6 city blocks from where I lived (and, seemingly, every 5th store was a hole-in the wall place you could get many grocery needs). I walked everywhere and didn't have access to a car, so that's my measurement - within ~10 blocks.
2000s: When I lived in Richmond VA . . . . I do not have the foggiest idea what I did for grocery store stuff. Looking at where I lived on the map today, I see 2 Food Lions, a Lidl, an Aldi, a Target Grocery, and a Kroger within 3 miles.
1990s: When I lived in Harrisonburg VA, I had zero grocery stores and 1 'general' store I had access to before I dragged my decaying dying car to campus. When I got the car, I still got my groceries from that tiny on campus general store. So I do not know what was available otherwise, though I do know there was a Wallmart back then. Looking at the map now, I see 6 grocery stores within a short drive: Wallmart is the furthest away but within 10 miles (9.5 to be exact); a Kroger, 2 Food Lions, a Target, and an Aldi round out the rest.