Is pure oxygen necessarily wrong?
The capsule design was to use pure O2 at 3psi to give roughly the partial pressure of O2 in normal air while reducing the mechanical pressure on the hull.
The mistake was doing an overpressure test by adding oxygen rather than N2 or He, a mistake that any scuba diver would have spotted.
But it doesn't mean that using regular atmosphere mix is the best. It means we have to ship lots of useless N2 to the ISS, it means shuttle astronauts had to purge with pure oxygen in case they got decompression sickness in a high altitude escape (as if) and you have to make the structure good to 15psi rather than 3
Are the physiological advantages of living at near 15psi important?