I guess the main attraction for an AB would be the brand / design continuity with baby products. That's fine within the AB domain but the mainstream brands would not dare go there. It's theoretically possible that a maker like P&G would design a continuous range all the way from newborn to adult, with some common feature, but in order to avoid any 'inappropriateness' there would have to be a transition to adult styling, and they would have you believe that their adult customers wouldn't want any association with the baby products via the brand. So basically it is a non-starter - the only association for the benefit of the AB would be that the diapers are part of that baby-to-adult range.
For the DL, there might be some benefits because there is an incentive to make the adult end of the range really good. Baby diapers are the big seller, so it would be commercial suicide to sell a low-revenue adult product line under a famous diaper brand name that makes adults question the performance of their baby products. They also have the greatest R&D budget, the best access to top quality production facilities and the best QC, so you might get a really good value and high-performance diaper even if it's not styled like the baby versions.
I would be interested to see what would happen if the reverse were true. Suppose someone made a toddler version of the Betterdry or Space.... how would parents and toddlers like it? Would it reveal that the current trend in baby diapers is just marketing, and what they actually want is a 'proper' thick plastic-backed nappy that little ones can really feel safe in?