I’ve been to the top of Pike’s Peak (or Pikes Peak, originally El Capitan); 14,115′. Named after Zebulon Pike, an explorer.
I had no problem there. People worked there all the time in the tourist center and in the Army’s research facility. Anyone’s car will go up the mountain. The thin air contains only 60% of the oxygen available at sea level. It’s chilly, rarely above 40 deg F in the summer.
People spend their entire lives up in the Andes, where they are inhabited to well above 10,000′. There’s even a small lake at 20,960′ on Ojos de Salado (22,608′). A Jeep was driven to 21,942′. People have ascended to the top of Everest (Sagarmatha or Qomolangma)(29,035′) without oxygen. So it’s not unreasonable to have people living in unpressurized buildings 9800+ ft. tall (in addition to the elevation of the ground where they sit).

From Tsui’s site:
“densely populated”. Strike 1.

Air pressure above 6000 ft? He doesn’t say that it’s enclosed (rather, “… open garden areas on each floor… all floors have specially designed windows with aerodynamic wind cowls, so that windows can be opened without having to resist tremendous wind forces… All windows in the building are operable so there is no pressurized, forced-air system used internally”)

Denver is OK, but not too many people live and work above 7000 ft. Strike 2.

I hear that La Paz is at 11 kft – population over a million, so I have to assume that you get used to it.

On the other hand, I’d like to see the tower built.