It’s tempting to reduce walkability to a simple arithmetic problem—a matter of having sufficient density to support amenities like a grocery store. But density alone does not a 15-minute neighborhood make. If you take the Sim City approach and separate uses from each other—commercial cluster over here, residential high-rises over there—you may still fail to produce a place where people can functionally meet their needs without driving.
A 15-minute neighborhood may be dense, but the more important thing is that it’s fine-grained and truly mixes homes, businesses, and public spaces seamlessly instead of segregating them into zones. This is why we need to let all our neighborhoods thicken up incrementally, instead of building clusters of high-rises to meet the demand for new housing.