According to this article, at 25,000 feet you have 3 to 6 minutes of “useful consciousness”, after which death can very shortly follow.
The partial pressure of oxygen at that altitude and higher is so low that the tendency is for the pitiful amounts of remaining oxygen in your body to diffuse out of your lungs into the surronding atomosphere. In other words, it sucks the life out of you (yes, a bit of hyperbole, but not too far off the mark). There is an altitude above which even unpressurized 100% oxygen will not be at a high enough pressure to enter the blood stream. I think about 3 lbs of pressure is about as low as you can go and still function reliably with 100% oxygen. I’m not sure where, exactly, that altitude is but it wouldn’t surprise me if civilian aircraft can reach it or get close to it.
If you reached that altitude rapidly (i.e. such as during the ascent phase of a flight) you will also be suffering from the bends as the nitrogen in your bloodstream comes out of solution. Gas in the intestines will expand considerably, causing both extreme pain and extreme episodes of farting and belching. Any pockets of relatively high-pressure air trapped in the ears or sinuses will likewise expand. This can lead to things like blown eardrums and tissue damage in the the sinuses. Any untreated dental problems like an abcess (even one not far enough along to cause pain at sea level) or dental filling that traps air can lead to excruciating pain, or even rupture of tissues or the breaking apart of the tooth in question.
Personally, if I was experiecing explosive farting and belching, suffering from the bends, having blowouts in my ears and sinuses, and having eruptions of poor dental work I’d prefer to be unconcious.
Not to mention the lethal cold. The lapse rate of approximately 3.5 F or about 2 C per 1000 feet has already been mentioned. Street clothes will not be sufficient.
Now - someone might bring up folks who live at high altitudes and who climb Mt. Everest at about 25,000 and survive. Well, yes, the human body does have some capability to adapt to extreme altitude, but 25,000 is really about the upper limit, and even then, only for short visits. Long-term survival at such altitutdes is not possible. And someone living near sea-level in Jamaica or Nigeria will not be acclimatized to 10,000 feet, much less 25,000 or the even altitudes airliners routintely fly.
Clearly, these stowaways are not familar with the above facts. Regrettably, these sorts of stowaways have been leaping aboard and dying in wheel wells for decades. It’s nothing new. It’s getting more attention now because of increased security, that’s all. The miracle is that some folks have survived these journeys - perhaps they were physically better adapted to thin air than average, or the aircraft flew lower than a lethal altitude, or some other quirk allowed survival - extreme hypothermia can lower body metabolism enough to allow survival in low-oxygen environments for a brief time period.
Now, something like the flightsuits worn by fighter pilots around the world would allow for survival in such circumstances, but such specialized attire is generally unavailable to such poor refuges as have, in the past, tried this route of immigration.
Then again - if someone did have sufficient protective gear and a bottle of supplemental oxygen, and successfully made such a wheel-well trip without detection… how would we know about it. hmmm?