In the graph below, I show the major league leader in strikeouts per 9 IP in each 5-year period, starting with 1901-05 and going up through 2003-2007. The minimum IP for each 5-year period was 750. Feller's 8.15 through 1939 was the highest up to that time and would not be passed until Koufax had 9.48 through 1961 (Feller did not pitch until 1936, so that 1939 figure is over 1936-39, but he had over 750 IP. His 7.92 through 1940 was the 2nd highest up to that time. And even up through 1943 he had 7.19, which was the highest since 1908 not counting his own performance. It is not until the late 1950s that even that is surpassed. You can see that things drop off pretty quickly after Feller went into the military (in 1942). The line does spike up just a bit in 1948. But that is from Feller (that is the first year after he got back from the war when his 5-year IP total reached 750). You can click on each graph to see a bigger version. (Where it says 1939-43 below, it refers to 5 different 5-year periods, with one ending in 1939, another in 1940 and so on).
In the next graph, I show what the line would be look like if I replaced Feller with the next highest totals over each 5-year period. The sections in red represent the next best totals and the green lines represent what Feller actually did. You can see that the trend would have been much different without Feller.
The next graph shows the 5-year leaders in strikeouts per 9 IP relative to the league average. A 200 means a guy had twice the league average. Feller is not as big a spike as Vance, but no one has approached what he did since then.