Everyone would have died of starvation or dehydration long before they reached the start of the stretching part, much less the end of the hole... unless A Wizard Did It, which would also explain how could somebody carve the holes to start.
A Wizard Did It would also explain how they could stretch so thin in the first place. Bones don't squish or stretch. However, this may be why Nakagaki got stuck: he was one of the unlucky (or is that lucky?) ones for whom the magic didn't work.
It might be worse for him: what if you have to force every single step once you start to get stretched?
What we see of Nakagaki, however, was technically just a dream of Owaki's. Nakagaki's situation may have actually just been "normal."
It doesn't seem like there's anything preventing people from just moving their arms forward, since most people's arms get narrower as they get further from the shoulder. Even if the holes really are carved that perfectly to people's body sizes, the body is flexible enough that you should be able to pull out your arms. After that, you might be able to get out your legs, and from there, all it would really take is turning yourself around. Of course, most people were likely too rapt by the holes to consider doing this, but someone like Nakagaki probably doesn't want to be there anymore. Getting your head out would be difficult, but maybe it wouldn't be as bad if you could at least move your arms around a little...
The narration during the second dream did state that the holes were somehow made that it was impossible to go back to the entrance. How this worked, exactly, wasn't explained, but presumably it kept the people in the holes from walking backwards or turning around to get out.
Yet the miner who went in to try and get Nakagaki out was able to go inside and come out when he wasn't able to find him.
It's possible that it only works that way for your own hole- since it wasn't the miner's hole, he could get out.
The worker was also about a head shorter than Nakagaki. It seemed like he was specifically chosen for the rescue because he was the smallest. And he probably entered and crawled strictly by the torso cavity.
The worker also had a rope attached to his belt that could be used to pull him back.
Also also, the rescue worker didn't put himself in right side up, spread legged like Nakagaki. He looked like he crawled in body angled forward, belly down on the part that would stick up into the crotch if you went in the intended way. So he could be pulled back out with his belly down. Might just get a little stomach chafed on the way out.
I realize this isn't what horror is about, but exactly what good does it do anyone or anything for noodle people to exist?
As mentioned previously they were created by the Japanese Paleolithic as a punishment for a "horrible crime." It's not that it does anyone any good— it's that it does the noodle-people a whole lot of bad.
All criminal penalties serve two purposes: punishment and deterrence. Getting put through one of those holes is quite a punishment, and seeing the end result walking around would certainly be an amazing deterrent.
The geological structure presented is a fault. It's a location where the rock has separated along a plane, one side pushing up to give a smooth surface. The fault occurred recently. So logically, there would be a rock facing underground that would have lined up perfectly with the exposed face before it split. If the holes aren't man made, it stands to reason they continue underground along the original whole volume into who knows what shape. Perhaps a shape men once held.
Once it became obvious that people were actually entering the holes, why didn't the authorities cordon off the area and seal the openings up with concrete? Yes, the site's of archaeological interest, but it's not worth people's lives to investigate.
It seems like whatever force is behind the holes in the fault line is intelligent and malign. If it's able to force people to go into the holes in the first place, then it can probably get the people who should be guarding it to continue allowing access, however reluctantly. The rescuer who enters Nakagaki's hole has to leave after going in five meters, where it doesn't seem like Nakagaki runs into anything significantly different than his normal body shape until he's several hundred meters in. Whatever controls the fault doesn't want anyone to stop what it's doing.
Anyone sent to cordon it off probably spotted their own holes.
Also note that the story takes place over the course of two days. Day one, people arrive and Nakagaki enters his hole, starting the panic. Day two, the disappearance of Nakagaki is still fresh, and a news crew is actually in the middle of reporting it when someone else climbs into his hole, which triggers several others to enter THEIR holes. Authorities very likely DID set up blockades around the site the next day, but Owaki enters his hole on the second night, and the story goes straight to a time skip after that.
What stops people from drilling that thing open? Or stick a bunch of dynamite down the OTHER holes?
Setting off large amounts of explosive along a geologic fault has been considered, but never tried, as a means of relieving tectonic pressure without a massive earthquake. The reason for the hesitation, naturally, is that you're forcing a sizable chunk of the Earth's crust to release a lot of stored kinetic energy, which can be rather nasty to anyone and everyone in the area. So, yeah, not so much dynamite. As for blocking the holes, anyone who's already seen their hole will not be deterred — Yoshida removes Owaka's barrier to enter. That said, it might at least prevent additional cases from occurring.
Arguably, the barrier Owaka built was simply a pile of stones stacked up to close the hole, which are easy to remove. There are way better methods to seal the holes if needed should the government order such barriers, and if one can't open their own hole due to lack of tools, they could be safe even if they saw their hole.
So suppose you're really fat. Your hole is also going to be wider — in the art you see holes of different body proportions. So, if you lost a bunch of weight, would you no longer have the urge to go inside, since it doesn't resemble you anymore? If you did, you'd be able to change your mind at least for a while, right?
Or, conversely, if you're really thin and gain a lot of weight, would you still be drawn to your still-thin hole, which.....
Or maybe the holes change sizes based on your current bodily proportions?
It's possible the force behind the Fault knew what kind of shape your body would be in at the time you went to climb in.
I know that the story cuts off right when we see the end of the tunnel, so therefore there is no way to prove this. But what happened to the deformed people after they got out? Did they survive for some time, or did they die shortly after they got out?
as a punishment for a "horrible crime." It's not that it does anyone any good— it's that it does the noodle-people a whole lot of bad.