I wrote this controversial tweet on the matter a while back (embedded here for convenience [1], controversial because it uses averages!) Which might make things seem more or less bleak depending on your perspective, but an important note from this data is that the 2017 median sales were about 2000 units per game (yikes). [1] https://twitter.com/gavanw/status/967249172804943872 Average number of units sold per game on…
2017: 49k, 2016: 36k, 2015: 37k, 2014: 39k
2013: 120k, 2012: 89k, 2011: 73k, 2010: 48k
2009: 35k, 2008: 27k, 2007: 75k, 2006: 48k
2005: 43k, 2004: (this really just means Half-Life 2 and Counterstrike): 820k
With the sales normalized by year, and noting that 2004 did not actually allow third party games on sale, there is not an appreciable drop or trend across the entire time range.. I suspect the crazy spike in 2013 can be attributed entirely to Dota2 and 2012, Counterstrike Global Offensive causing mean skew.