Back to the OP...my 3 cents.
One major crisis early in the BSA was to get all the many splinter groups to agree to come into one organization, One being the Rhode Island Boy Scouts which still exists today as a trustee organization.
Another crisis faced (this will be requirement number one of the new DEI MB), was the pushback of the YMCA to allow Catholics, Jews, Indians and "Negroes" and other ethnic and racial boys into the program.
In 1972 the BSA membership peaked at 6.5M youth. What to do about this? I know let's start the ISP. I was part of a 50 Eagle Scout group who went to Schiff to evaluate this new program. Everyone of us said it wouldn't work and we all know how that worked out. Membership has never recovered and could fall below 1M at the end of 2020.
The 18 year old cap for Eagle was set in 1952 when they did a major overhaul of the MB program. The BSA allowed those men who had gone off to WWII before they could finish their requirements to do so. This requirement was essentially ignored until 1965 when the BSA added the Troop Warrant Officer and Eagle Project. Since adults could not hold a troop youth position, that basically shut the door on adults although there were still councils who defied that. The exception to 18 is for Youth with Disabilities for which there is no age limit. The 18 year old cap is also since the BSA requirements are essentially written for 11-14 year olds.
Definitely regional. Like most things with advancement, some councils set a particular tone, and eventually it was echoed nation-wide, until it was codified. (Which directly violates my Rule #1: Don't ask for a rule. You'll live to regret it.)
I personally don't see the need for a special designation. There aren't going to be a lot of adults who do go all the way to Eagle, which is already a "silver" award. They just get to be called Eagle Scouts like the rest of us. But, more to the point, we would strongly encourage everyone to be 1st Class Scouts. The patch would synchronize with the concept. Most every adult would make an effort to at least nail that. And yes, it's a shame BSA has oversold Eagle almost to the exclusion of this "middle" rank.
One side effect: there might be fewer youth who make Eagle because, lacking a deadline, the natural procrastinators will keep doing what they do. However, I think youth who see new adults (their moms and dads, even) struggling to master 1st Class skills will be inspired.
My experience is with renewing BSA Guard, it just gets harder to knock out those sprints every three years, and one particular year I came back defeated a couple of days in a row. That third day, the few scouts from my troop in the aquatics area cheered me on when I finally nailed it.
I think this is an area worthy of discussion regarding how it may impact scouting going forward. Extended adolescence is an issue. Youth do seem less able to handle certain responsibility markers that prior generations were more adept with. However, youth today have greater and different pressures than older generations did. Just watching a 14 year old trying to navigate the virtual learning environment has been mind boggling for me. Can I depend on him to load the shotgun, hike 10 miles in snow, and bring back dinner? No. He'd be crying. Can I depend on him to figure out how to jury rig a hotspot when the internet goes down so he can still make class and I can work and bring home dinner? Yes. Different life skills, but just as responsible.