It is not difficult to do as you suggest in your last paragraph. It just isn't the most efficient use of time. One way is not "better" than another. They are simply alternatives that get you to the exact same place at the exact same time. My suggestion is not designed to get you on the ride any faster. It is designed to pull fidgety kids, tired legs with bulging veins and people who don't deal well with lines out of the lines until the last possible moment. For eons, WDW has allowed people in wheelchairs to join their group when the rest of the group gets to the front. No one ever complains about this. Why? Because the person is in a wheelchair? It isn't polite to complain about such things? But if an elderly person has swollen feet, or a ADHD or autistic child is having difficulty with the lines, shouldn't we do the same for them? And if we do and don't complain, aren't we acknowledging the fact that they didn't gain any advantage. They simply waited in line a little bit differently than we did. You see. We already have such a system in place for certain guests. To think that such a system could not work for all guests is illogical. Granted, some queue modifications would be needed. But from a purely theoretical standpoint, which is all that this is, the system would work, both from a mathematical and societal basis.
I definitely see your points. The issue I can see (other than queues and spaces not being set up for this method, at this point), is could/should there be a limit on number?
To use your examples...if one person is ordering food for a family of five, it makes no difference to me whether they stand in line alone to place that order, if they all stand in line the entire time, or if 4 of them (or 1 or 2 of them) return at the ordering point. Actually I think the last option is probably best - streamline the line, but they come help carry the food, etc.
However, I DO have a problem with it if those 4 people return at the ordering point and then all place SEPARATE orders, paying separately, etc. Because that does impact my experience in terms of waiting time, etc. I may not have chosen that line had I expected 5 people in front of me (plus any other groups) instead of that one. Another line may have been much shorter. (I have actually had this happen to me before, BTW, at Cedar Point. 1 person in front of me, after quite awhile in line already, suddenly became 4 people - all placing separate orders, digging out cash, etc. I was quite irritated, as were those behind me).
So if a ride accomodates 3 in a vehicle, assuming things are set up properly, I would not mind if for a group of 3, 2 of them waited elsewhere. But if the group size is 13 (size of our group last trip), it is a problem if only 1 person waits. People behind cannot judge accurately if they want to join the line. A group of 13 takes 5 vehicles. Multiply that by numbers of groups and a line that looks quite "short" or reasonable, very quickly becomes a VERY long line/wait.
The only solutions I could see to this - a policy that the number of people actually waiting in line has to match the number of ride vehicles needed (5, in my above example). That's not very realistic, how would it be enforced? And what if you don't KNOW how many a ride accommodates? Etc.
Or increase staffing to track group sizes and keep an ACCURATE wait time posted at all times. So no one would be looking at the "line length", just at the posted wait time, in making "wait or not" decisions. This seems more realistic, and of course would have to be done in conjunction with queue/building changes.
I have not seen the new Dumbo yet, it opened just after our last trip, but it sounds like this is along the lines of what Disney is doing? Make your decision based on wait times, there is no actual "line" to guess by.
*** I will add that because Disney is NOT set up this way now....groups should enter the line together and wait the entire line together. Only possible excuse I see would be a very small child with parent needing to leave the line for bathroom/diaper change, then rejoining. Already IN the line when the "emergency" happens. Or a baby that just starts crying their head off, I can see that too. NO reason for anyone not already IN the line to be moving ahead.