Seniority System
When applied properly, the system should be used only to dictate the sequence in which pilots are asssessed for command, if the most senior candidate fails to come up to the required standard during training/assessment then he/she will not be promoted.
Given the length of time command slots take to come around these days - plus the high standard of co-pilot we are seeing in all three companies (only the most motivated can make it through the tortuous licencing malarkey) - failure is uncommon.
Wiz