I have an half acre lot with a house on it. Within a 15 minute walk of my house I also have a train station, offices, a bank, a grocery store, restaurants, bars, salons, tailors, a medical clinic, and parks. Within a 25 minute walk there are also schools, additional train lines, museums, more parks, hardware stores, offices, a courthouse, and many, many more restaurants, bars, coffee shops, retails store, galleries, parks, boutiques, B&Bs, etc.
Depending which train line you take it is 50 minutes or 25 minutes to the heart of major city on the northeastern seaboard. It takes about 30-45 minutes to drive to the center of the city by car. It is a 15 minute drive to the airport.
What "15 minute cities" try to create is what exist already with most towns and neighborhoods in the USA that predate the automobile. My town was first settled in 1647. The first passenger train here was in the 1850s. You would be so lucky to have everything you need within a mile and half from your home. What you seem to forget, is no one is going to force you to stay within that mile and half radius. I am in fact able to leave whenever I want. I just get to decide if I want to do so on foot, a bike, a car, or by train. Over half the people in this country have to jump into a car to go to those things I can comfortably walk to.