In a walkable city you don't need to go from one end of the city to the other. 10 mi. is insanity. You should be able to go a maximum of 2 mi. and find what you're looking for. I'm in a biking-density neighborhood, and have multiple grocery stores, convenience stores, restaurants, and furniture stores within 1 mi. of me. In a walking density neighborhood, I would expect that to all be available within 0.5 mi. of me.

Nobody needs to do much of anything, but people want to. 10 miles is 130th st to downtown Manhattan, a distance people most certainly either want or need to travel regularly. Even in the most culture/store/apartment dense corner of the country.

Now I'm all for effective public transit and dense neighborhoods, but walking and/or biking are simply worse for a lot of use cases.

I'm not familiar with Manhattan, are you describing the phenomenon of building dense commuter urban-areas and far-flung residential areas? Because that is not part of being a walkable neighborhood.

But what stops public transit from being the solution here? In fact, over a 10 mi. radius, public transit scales much better than single-occupancy vehicles, and even fully loaded sedans. Go to Tokyo and folks take trains all day. If you're taking about low-to-medium density areas (which isn't a city) then yes, cars are the most effective.