Moving from the decline in "ultra-quick hooks," we are best advised to now look at the overall distribution of starting pitcher inning ranges for all games started over the past century. (Again, thanks to Retrosheet and Forman et fils for developing and "accessible-izing" this data.)
We've color-coded this chart to show the direction of change--orange for when the percentages are rising, and yellow for when they are declining. We added a little "color intensity" (and here's to "Fright-Quote-R-Us-dot-com," the folks who "sponsor" our merry little adventures....) to certain of the data cells to show a more rapid rate of decline.
The bold type shows which column has the highest percentage of games in each "time zone." As you can see, the "baton" gets passed from 9-9.67 IP to 7-7.67 in the 1980s, and then moves on to 6-6.67 IP in the 90s, which is now consolidating its lead as we move forward.
Next up: winning percentages and all that "rot."