The point is, in a walkable city, nobody needs the speed. What's the point of driving at 40mph in a 10x10mi city vs using a scooter at 15mph? You'll probably hit a stoplight after hitting 20mph, and then waste braking to stop again.

Electric mobility scooters are, per-mile, much much cheaper than cars, and cost much less to purchase.

In your example a corner to corner trip would take ~90 minutes by scooter, considering the 20 mile Manhattan distance + stoplights. A ring highway would get you there in 15-20 minutes. Even a set of properly timed lights would allow 35mph traffic to cross the width in ~20 minutes - which notably can't be (or just never are?) efficiently timed for pedestrians/scooters.

for the extra hour+ that scooter would take (given no/low traffic) there is no price where its worth it vs the ~$10 that trip costs in a car. (20 miles, irs rate of ~50c/mile)

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.

Car infrastructure is anything but cheap. Our suburbs are going broke, in a large part, because maintaining roads, bridges, parking lots, and solving drainage problems caused by these things is very expensive. Add to these the external costs of pollution and car crashes, and it gets even more absurd.

Most electric wheelchairs have 10+ mile ranges,can be fully recharged for pennies, require even less physical exertion and special skills than cars. And they don't put other residents at serious risk.