"But I'm still waiting for a non-speculative justification of why you can't lie on the floor when the plane is cruising along smoothly and passengers are allowed to wander about the rest of the plane."
JPa, your question is kind of "ridiculous"...!
Note that it's an absolute basic in safety situations (such as, oh, I don't know, flying 300 people through the air, no wait, through the upper atmosphere where there's no air to speak of, at the speed of sound!!!) that you have to have order and keep things - let's say - ship-shape.
In any safety situation it is not the case that "anything not explicitly forbidden is allowed" (as is the case in "life generally"); in safety situations it's very much the case that "only things explicitly allowed are allowed".
You could make a dozen really crazy, whacky suggestions of things that have "never been explicitly ruled against" in flight ... say, "why can't I do hand stands, only in my floor area, during a flight?!" .. in each case, as a sort of silly exercise, you could explicitly list all sorts of reasons why that is a bad thing to do.
But in safety situations there's a blanket idea that "you can't do weird shit," as well there should be.
As a curiosity: some friends of mine (foolishly) let their kid sleep on the floor (in front of the seat as you describe) on a Swiss flight about 5 yrs ago. The staff were incredibly angry and it almost reached the point of alerting the authorities on arrival. (I'm unclear if "sleeping on floor" is explicitly forbidden on Swiss - but why would it be or why should it be? "Anything unusual" is forbidden in safety situations.)
So the very simple answer to your question is "you must keep order" in such a dangerous situation as a flight.
Note that, very simply, if you literally just simply move around an awful lot during a flight - constantly walking up and down each aisle etc - you'll be told forthwith to stop that and sit down. (You're not, by any means, at liberty to randomly wander within your class on a flight; you're to sit the hell down and maybe go to the toilet if necessary; "stretching your legs" is tolerated.)
If I was on a flight where the cabin staff were allowing people to sleep on the floor between aisles, I'd instantly get the pursor (or whoever's important) and have the whole situation corrected, and then take legal action against the airline subsequently.
If you literally want someone to list the dangers of sleeping on the floor...
(*) obviously you need to, generally, stay seated during the whole flight so that if you need to go to an emergency position, you're as close as possible to that situation.
(Note that this is precisely why they tell you ......... to stay seated during the whole flight!)
it would be madness if, when the captain declares seat-belts-on, or even an emergency, the staff had to run around and find everyone sleeping on the floor and have them get up and sit in their seats!!
(*) in case of sudden turbulence, being seated properly (even with no belt) is spectacularly better than being laying down sleeping, with your eg. face inches from a whole lot of sharp steelwork, bolts etc.
I.E., very simply, laying on the floor between airlines seats (I mean, when the plane is parked at the Boeing factory unmoving) is just not sensible or safe, it's not somewhere body parts are meant to be
(*) further to point (i), if you had ten people asleep as you describe and there was an emergency, 8 of them would get tangled (clothes or body parts) on all the stuff down there in rushing to come awake and get back to a seat.
Anyway, "still waiting for a non-speculative justification", I'm not sure if you were just kidding around, but there's three justifications typed out (please tick my answer! :) ) one could easily type 4 or 5 more.
in general ..
(*) the reason they say "remain seated for the whole flight" is obvious! its the safest if sudden turbulence, and, the quickest to get to strapped-in if there's an emergency
(*) it's just an outright bad/rather crazy place to be (get scratched, caught, your hand flops out in the aisle while snoring, scalding liquids tipped on you, etc etc etc)
I hope this answers your fun hypothetical-like question!