My kids are ages 16 and 19, so at this point I doubt I'll run into this problem, but if I did...
At their present ages, if they were buying diapers with their own money, wearing them privately at home, keeping themselves clean, dealing with the trash, laundry, etc., I'd stand aside and let them be responsible for their quirks. It's hard to see where my interference would make anything better. On the other hand, if they were newly potty-trained preschoolers and were asking Wife or me to put them back in diapers, that would be a hard "no." It's tempting to project my own fantasies onto this scenario, because I've been that little kid who desperately wanted to be back in diapers. As a parent though, it would be irresponsible of me to do that. When possible, kids do need to be potty trained. Allowing them to wear diapers indefinitely, without a medical need, is socially limiting.