In the 1980s, QF command upgrade occured at ~18 years in the compay. At the domestics (Ansett and TAA) it was around 10-14 years. Some of these people needed more training but anecdotally, the failure rate was pretty low
Another point of view is that the long wait for a command in some of the long established carriers (PANAM, for example, as well as the Australian carriers mentioned), was because simply there were no command vacancies. Pilots were on huge salaries and very few left earlier than compulsory age 60 or less, retirement. So, no movement in the ranks meant no upgrades.
It certainly didn't ensure better pilots despite greater exposure. In fact the morale among the long term copilots was never that good. Like Peter Costello waiting for John Howard to get another job.
Also, the pilots' unions protected those incompetents who were unable to make the upgrade and thus we saw the "professional" first officer who built up unassailable seniority because he could bid not to work that month.
The relatively high failure rate for upgrade in certain airlines may be due to lack of flight derck management and flying skills. But there is little doubt it can equally be traced to the different checking styles of check captains rather than the incompetency of the candidate.
Some check pilots should never be in the job. Ever wondered why the airlines don't require their check pilots to undergo psychological checks for suitability to be a check pilot? Answer: They wouldn't pass the tests