It could be purely artificial. For example, there could be a data selection bias by including or excluding different leagues in the different time periods. Indeed I have much more leagues covered in the latest years starting from 2007 than I had in the 1980s. However, there are no discontinued national leagues. Every country's top national league that was once covered is covered until today. So there are only additions of leagues and no removals. Any addition of leagues, however, will broaden the base of players to select from and will increase the likelihood of a great player in the sample. Adding leagues should therefore lead to an increasing number of players in the list instead of a decreasing.
Another artificial source of the Dark Age could be a quirk in the Goalimpact algorithm. It is difficult to argue for or agains that. Unless the cause of a potential quirk identified, it is just hypothesizing.
A potential alternative explanation was given by an anonymous reader in a comment.
Ich schätze damals war man halt im vergleich zu heute mit 21 noch nicht zu alt ;) Denke es liegt daran, dass man in dieser Zeit einfach erst in einem höheren Alter seine Einsätze bekam...
I figure, back then 21yo players weren't considered as too old ;) I think it is caused by the fact that players got playing time a higher age those times.
Following that line of thought, young players were less good between 1987 and 2007 because they couldn't play at matches top level and hence developed slower. I like that hypothesis because it is testable. So here is the chart.
As comparison, the blue line shows the average minutes players have played already in the years before they turned 21. Here we see a drop that coincides with the drop in Goalimpact at the end of eighties. When it used to be just below 1500 minutes, it dropped blow 1000. It is a drop of nearly 40% average playing time. It never picked up again until 2007, but this increase is, at least partly, because of the addition of various youth leagues to the database.
So the anonymous reader may have a point. At least the drop in Goalimpact at the end of the eighties might have been caused by the football clubs. They didn't give enough playing time to young players anymore. As a consequence the players may have developed at a higher age than in the previous decades. The big teams rather bought 'complete' players from selling countries than to educate their own youth.
However, the drop of average GI in 2000s is clearly not explained by this. Here I assume the addition leagues to be the main cause. The leagues added are mainly second tear national leagues and the average player playing there will have a lower Goalimpact than in the Big5 leagues that have previously dominated the sample.
The recent increase of the average GI, in contrast, can't be explained by the addition of new leagues. So, if indeed the effect is real, and if players did develop slower between 1987 and 2007 than before, we have an indications that this 'Dark Age of Football' is over by now.
Appendix: The data for the chart
|Year||Number Players||Avg Goalimpact||Avg #Games||Avg #Minutes|