Era-weighted goalscoring

The above chart shows average goals per game in the English First Division and Premier League between 1929 and 2015. In the 1960s the long run average fell from over 3.3 goals per game to 2.7 which has persisted for almost 50 years. The only factor that can really explain this is the rise of more defensive tactics. Until the 1960s formations were based on "WM" - the M representing two fullbacks and a centre-half with two wing halfs playing upfield and the "W" two wingers and a centre-forward supported by two inside forwards. The first of the now hegemonic numerical formations was 424 with one of the wing-halfs withdrawn to the back line. The Italians pioneered the "sweeper" - one of the back 4 plating slightly behind the other 3. English football has always preferred either the twin centre-half model or a defensive wing half. All of the subsequent permutations that the talking heads go on about ad nauseam have made very little difference, it seems, compared to simple move of 4 defenders instead of 3. I don't have any good answers for the decline from 1939 to 1950 and subsequent rise; food for thought. Among the many observations that may be made from this one that occurs to me is a re-evalaution of Alan Shearer. The all-time record for 1st Division/EPL goals in a season is Dixie Dean's 60; this would be 38 in the modern era. Shearer scored 34 in 93/4, part of a three-season run of 31, 34 and 31. (Andy Cole is the only other player to have scored 34). His career total of 283 EPL goals would have been 370 in Dean's era.