I mean, that’s kind how it is where I live. I live in a 1400sf home on .23 acres of land. I’m five blocks from downtown, where there’s businesses, a courthouse, a train station, thousands of apartments. All the schools are walkable. Parks are walkable, with amenities like pools/splash parks, playgrounds, a paved trail network. We fit about 6,000 people per square mile, which is pretty dense.
I don’t think it exactly fits the 15m city concept, because I don’t think there are enough jobs in town to support everyone, but it’s a pretty good mix. A variety of housing types is important, simply because people want what they want, and I think it makes a more cohesive society to try to have something for everyone.