I’m sorry, I must have missed the part in the original trilogy where they show how quicksand in a desert is against the rules.
Oh, no. It’s OK. You didn’t miss that, as there wasn’t any contrived quicksand in the OT.
But as explained elsewhere in this thread, the “dry quicksand” element is contrived because the planet Jakku is literally littered with junk from previous battles that has not been eaten up by any quicksand. So here we have a case where a fantasy isn’t bothering to follow its own rules.
I know, I know. Let me say it for you so you don’t have to waste keystrokes: Poe & Finn’s TIE fighter just so happened to land on a spot of dry quicksand…and it’s sort of “delayed-acting dry quicksand.” That is, it only activates at the key moment a main movie character comes upon a vessel sitting upon it that needs to be conveniently gotten rid of to move the contrived story along.
So what we really have is this: If the TIE fighter is crashed, logically (again, here we go, trying to apply logic to this mess), Finn would climb into the wreckage to find Poe, the guy who just helped him escape certain death. Upon not locating Poe’s body, Finn, a decent human being, would be compelled to, you know, go look around the desert some to try to find Poe – after all, maybe he’s still alive somewhere and desperately in need of help. But no. We don’t want to waste valuable screen time with all that logic mess, so we’ll just have the desert swallow up the TIE fighter and “explain” it away by having another character babble something about quicksand earlier.
OH! But wait a minute. Finn found Poe’s jacket. This would give him evidence that Poe survived the crash and took the jacket off because it’s hot. Right? So, logically, wouldn’t Finn conclude that Poe is still alive? And, you know, probably wandering around the desert, hoping to find Finn? Nah! Let’s put the jacket on in an arid desert, forget about that schmuck Poe, go wander down to the settlement, and then tell everybody there that I meet that Poe didn’t make it – even though I have good reason to believe he did make it.
You really need to go back and watch the film.
Finn gets to the crashed TIE , approaches the cockpit shouting for Poe, moves the jacket so he can look inside the cockpit, but the moment he tries to look inside the ground begins to give way and the TIE sinks. He doesn’t get a chance to climb inside. This all happens within a few seconds. Finn shouts out for Poe as it is sinking. He believes he was trapped inside the ship and dies. So why the hell would he go off in search of someone he believes is dead?
It was established prior to this that there is an area called the sinking fields when Rey tells BB-8 he needs to avoid that area. Its never referred to as quick sand. For all we know the sinking fields are created by a creature burrowing under the sand causing the surface to destabilise because of the tunnels. Notice there is no junk in that area at all. Just because the TIE sunk in that one area does not mean that all the other junk littered around, in different areas, would also sink. How is it any less plausible than someone being able to move objects with their mind, or any less convenient that the ship lands in those fields as it is that they happen to stumble upon the death star so they can rescue the princess?