I know it’s powerful (this IS a Lucid Dreaming forum), but I also have a functional (and technical) understanding of it’s various shortcomings, one of which is keeping track of multiple ‘items’ while your concentration wanders freely. If you stop thinking about something, it’ll probably stop happening in the dream. I know what you’re saying about our mind getting used to rules and applying these same rules to our dreams automatically (simply because we’re so used to them) - but the mind just isn’t powerful enough to maintain them at all times.
In RL we have the physical world itself constantly moderating our actions, even when we don’t remember to consciously consider the rule prior to our attempts to defy it. I might absent-mindedly try to pick up something far too heavy for me to lift, but low and behold, I’m quickly informed by the unyielding and permanent rules of our reality that it’s not going to happen. In the dream, however, there’s no underlying rule stopping me from doing this. My mind is only going to prevent me from lifting the heavy load if it expects that I won’t be able to.
For that reason, a dream lasting several days (though I don’t believe that’s physically possibly) would be inconsistent, random, and after a little practice, almost entirely under your control. Above all, it probably wouldn’t be realistic. Our brains depend on external stimulus to maintain stability and consistency. A dream can only remain so true to RL.
Jack: I’m not too familiar with psychological disorders, autism, and so on, but I’d be inclined to believe that those people you spoke to had some form of disability that provided the seemingly lengthy and scary experiences. I’ve heard stories from people who have had sleeping disorders that prevented them from entering deep sleep, and instead caused them to experience REM throughout the entire night. This gave them the impression that they had dreams lasting months, and at various points they felt that they had control. They were also sincerely depressed and fearful of going to sleep.