I think the key element there is "as elderly adults". I haven't read the book (tried, the Nadsat took effort) but the movie is one of my favorites. If the ending had Alex popping up from his bed shouting "I want to be good!" I would have filed the movie away as Spielbergian feelgood schlock (see also: AI).

I do think people change, but it happens slowly, not quickly. Kubrick's ending still leaves us with the ability to imagine an older, wiser, regretful Alex. But you can't credibly force that into his youth, and the movie is already 2 hours 15 mins.