"a long peace must be evidence for a decrease in war risk, because a long period of many wars/casualties would be evidence for an increase in war risk"
I do not think this is true. Perhaps you are confusing the incidence of war and the risk of war. A long period of many wars would be evidence for an increase of war, not an increase of war risk.
At about 1:00 in this video NNT explains how a small amount of benefit also exposes you to an increased risk; the corollary would be: an increase of peacetime exposes you to an increase risk of war. (The video is meant to provide the intuition, not a proof.)
I do not think this is true. Perhaps you are confusing the incidence of war and the risk of war. A long period of many wars would be evidence for an increase of war, not an increase of war risk.
At about 1:00 in this video NNT explains how a small amount of benefit also exposes you to an increased risk; the corollary would be: an increase of peacetime exposes you to an increase risk of war. (The video is meant to provide the intuition, not a proof.)