If cabin presurization is lost, there will be no dangerous accumulation of CO2 period, because it leaks out through the non-airtight seal between the oxygen mas

If cabin presurization is lost, there will be no dangerous accumulation of CO2 period, because it leaks out through the non-airtight seal between the oxygen mask and your face. The emergency oxygen is more about providing a breathable partial pressure of oxygen (~0.2 atmosphere IIRC) when the total pressure is much lower than sea level, by greatly increasing the concentration.