I was in Scouts in the late 60's. I got my hiking merit badge. My 20 mile hike was in Morristown, NJ. I don't remember anything about 10 mile hikes. Maybe they didn't have them then.
Now I live in Colorado and backpack a lot. This year, I'm seeing a lot of kids on the trail. There is a 12 year old kid on the Appalacian Trail this year. She hiked the Pacific Crest Trail last year. It's not unusual to see kids 5 to 10 years old far from a trailhead with their parents. I met an 8 year old who is already dreaming of the long trails.
My personal opinion is 8 miles should be about the longest required for the MB. I have a few reasons for this.
1. It's easier to find adults who can hike 8 miles.
2. 20 miles is really a darn difficult hike on trails, so it leads to urban hikes. I like urban hikes, so I'm not knocking them.
3. Being able to do 8 miles is good enough for going backpacking. It's a more realistic distance in mountainous areas.
4. It's probably easier for a scout to legitimately plan an 8 mile hike. I would really insist each scout planned the hike even if an adult picked a trail.
With shorter hikes, it's more realistic to allow a scout to make all the decisions. Rest breaks, lunch, wading in a stream or standing under a waterfall. Perhaps it could be a requirement for a scout to lead a hike for the last one of the MB. (With adult supervision if needed.)