I have a lot of time to think, which is not as desirable as it sounds. Anyway, my idea is, what do we really gain by moving from diapers to underwear? After all, we lose the ability to relieve ourselves conveniently and discretely when and where we have to. Then we must face often crowded and unsanitary public restrooms, risk being caught if we go outside, or hold it until we're in pain. To those with shy bladders, public toilets can be torture. And then there are the poor folk who must bare shame and embarrassment if they can't hold it any longer and suffer an accident.
Of course, there are health and courtesy concerns. But if one follows common sense practices, diaper use will not harm or overly bother anyone.
So diapers are not just necessary garments for the very young and the incontinent, but can be practical articles for those of all ages. When you think about it like this, why should there be any stigma? I can only imagine the countless instances of physical and/or emotional discomfort that could have been saved with a diaper.