how anybody could live at 10K for long periods of time

Well in most cases towns that high only existed because of the mining activity during the Colorado Boom Days that started around 1860 and extended into the early 1900s, but you probably already know it was because of the mining. I lived in Leadville which is two miles high and was founded during a gold rush. I have been out exploring in the mountains all over this area and it's common to find old cabins way up near treeline along with mining ruins at like ~12000 feet. People would work and live at that elevation long term. Not surprisingly a miner's lifespan was often rather short.

I was still in grade school

I bought the lie till I was about 20. Wish it hadn't taken that long but I'm grateful it didn't take me longer.