The data of people taking longer to reach professorship should have included the average age of retirement of professors over the same period, which I would exp

The data of people taking longer to reach professorship should have included the average age of retirement of professors over the same period, which I would expect to also have shifted during this interval (and roughly by the same amount, as I know it did in the Netherlands). It explains why there are relatively fewer open positions for people to transition into, simply because the current cohort of professors isn't retiring. Not as fancy as a comparison to drug dealers, unfortunately.