I use to think the same thing but now I don't think it would be that difficult at all.
Think about it from this angle, I use to fit into a size 6 diaper at age 14, this was before the stretchy sides of todays baby diapers, these diapers were the last generation plastic backed luvs, and the hybrid pampers (plastic tape panel, cloth like cover for the diaper itself)
I grew out of those, but I never had any leaks due to cut, it was a capacity thing with me. Fast forward to age 26 or so and although I was wearing adult diaper one and off, I did retry luvs, these had the 1st generation of stretchy sides, had blues clues on the tape panel. They stretched to fit, they obviously had a lower cut but I still had descent coverage, no gaps due to cut and didn't even hold half a pee.
Now today what I'm seeing is a basic template of measurements of what the guts of an adult diaper should be, most are designed to have the tapes fasten around the hips and although some of us can due to body size have that classic belly button landing area for the front top sheet, they are designed to come up just a few inches higher than regular underwear.
A few simple changes of adding length, a few elastics and stretchy sides and you have a diaper that will look like a babies, move / wick / sag in the right spots while not falling to far below the belly button, and stay in the small of your back, plus hold an adults worth of wetting. The new bambino's come pretty dam close to hitting this, there just 1-2" short in length and the tapes are designed to be pulled straight across, if someone who had an auto cad 3d program could design a single wide tape, say the size of a sdk than then using elastics to create an even pull on the top and bottom of the tape mount they would hit the nail on the head, think of a triangle, you pull the v and the top 2 lines in the v move equally with equal tension, the elastic helps adjust for different body shapes.
