The conclusion I’ve come to is that about 99% of people can fly a plane at 10 or 12 thousand feet without oxygen, and have no issues. However, I feel significantly better after 3 to 4 hours of flying at 10 to 12 ft using oxygen even periodically, so I elect to use O2. That doesn’t mean having it on the entire time but just using it every 45 minutes for 10 or 15 minutes. Obviously at 19,000ft that’s a different story and it’s worn the entire time, however I land significantly less fatigued and exhausted, and actually have energy to do things even after a day of 9 to 10 hours of flying.
a cheap O2 setup can be found used for 300 to 500 bucks and will improve those long travel days significantly. There is also the option to go up high, and avoid icing, turbulence, weather, and catch some pretty nice tailwinds. Just understand the downside is you can dig yourself deeper into a bad situation too.
I guess all I’m saying is just because you can fly without oxygen at those altitudes doesn’t mean you need to. For an hour flight at 12,500, who really cares. If it’s 3 or 4 hours, the difference is pretty noticeable.