Originally Posted by klw
Not disagreeing with your premise at all but another important reason for the reduction of innings by starters is the demise of the 4 man rotation. Assuming an even number of starts among all the members of the staff, a 5 man rotation would average 32.4 starts per 162 games but a 4 man rotation would average 40.5 starts, a difference of 8.1 starts. Not way that starts can be manipulated an ace may get a couple more but most years the league leader in starts is at 35 games and none have reached 37 games since 1991. In the 1970's however the league leader was typically in the low 40's. (Historically it seems that this was a high period but 40 seemed typical in the 4 man era, remember shorter season pre-1961.) http://www.baseball-reference.com/le..._top_ten.shtml
Assuming 5-7 starts lost to the change in rotation size would reduce inning load from 30 to 50 innings per season without even getting into pitch counts.
Exactly right, the switch from 4-man to 5-man rotations is a big part of the issue. But why did they make the switch from 4 to 5 pitchers in the rotation? To cut the innings of their good starters in an effort to avoid overworking them.
League-wide innings reductions were not a by-product of switching to a five-man rotation, the opposite is true. Five man rotations were a by-product of teams wanting to reduce the workload of their pitchers.