Parents need reliable childcare. A teenager or aging parents can't fill this gap reliably. I need somebody to watch the kids most days from 8 until 4. A high schooler won't be able to do this. My in-laws who live 1.5 hours away and have mobility issues (plus their own lives) can't always do that.

I agree we need strong community in close proximity, but still people need to travel to destinations a few miles from their houses sometimes. Biking is a great (this is my primary form of transportation) but even I know biking and walking cannot fill every transportation need. We need the trifecta of biking/walking/transit to ween ourselves off of cars and develop denser communities with more people to interact with in our day-to-day lives.