I grew up in the middle of nowhere on a dirt road where people drove way too fast. I did not learn how to ride a bike until I was in junior high. My dad never tried to teach me. I'm not sure if he has ever ridden a bike in his life but he was also considerably older than most parents of my friends and just didn't have the energy to teach me... and I never really tried to learn because I had nowhere to go. If my friends and I were playing outside, we'd usually go off into the fields and woods to explore. A bicycle wasn't much use back there. One day when I was in 7th or 8th grade my oldest sister's boyfriend was over and when he learned I couldn't ride a bike he decided to teach me. He gave me a few tips and helped me steady myself while I was getting started and off I went... I think it took him about 15 minutes to teach me. Maybe it's just a lot easier after a certain age.
My parents were still a little apprehensive at first about me riding on my own on our road but they got over it pretty quickly.
I told our daughter that she'll be able to ride on her own (or with the neighbor girl) soon enough. Our neighborhood is pretty quiet but we live at the one spot in the whole neighborhood where a few roads come together at a curve and people sometimes drive a bit too fast, especially in the afternoon when the kids are leaving the Catholic high schools a block away. I want to see my daughter be a little more sure of herself and better learn the rules of the road before she goes off with her friend.
Our youngest is also lucky that she has a lot of friends that live in our neighborhood and the next neighborhood over. I think that will help motivate her to improve her bicycling skills. She'll be 10 next week and that's old enough that we don't mind if she explores on her own. We already let her
walk on her own to her friends' houses or to the park.
Regarding the full size trikes.... When I was a kid we'd go stay with my grandma in Florida a few times each year and she had a bicycle and a full size tricycle. Before I could ride a bicycle I would always ride her trike around her neighborhood and my sister would ride the bike. I remember the first time going down there after I could ride a bike and I told my sister that
I was using the bike to get to the pool and she could take the trike. After that I realized how much more fun the trike actually was but so did my sister and then we'd fight over who got the tricycle.

The tricycle doesn't turn as sharply but man those things can get some speed going!
There is some guy that appears to be my age who rides a trike like that around our neighborhood in the summers. I'd seriously consider getting one if I had a bit more storage space. And you are right that they are more utilitarian - you can put a pretty good size basket on the back and carry a lot more with one of those than you can carry when you're on a two-wheeler. I occasionally see an older woman at the grocery store near our house on a tricycle and she always has a ton of bags in her basket when I see her.