That does not support your argument that there is a adaptive advantage for reduced deadliness. The fact that it was exceedingly deadly to non co-evolved hosts indicates it was not the disease that became less deadly, but that the co-evolved hosts developed better defenses.

> Our bodies are full of bugs that have evolved into peaceful coexistence.

That is a argument that there is a continuous adaptive advantage to reduced deadliness down to ~0%. Again, Rabies had and continues to have a nearly 100% fatality rate in co-evolved hosts for thousands of years. Smallpox had a 10-30% fatality rate. Any magical inherent adaptive advantage for reduced deadliness failed to materialize to continue pushing down their deadliness.

Or put another way, a disease can have a 30% mortality rate and still do a really bang-up job at propagation with limited adaptive pressure to reduce that further for thousands of years. Peaceful coexistence is more likely a artifact of the specific dynamic than any sort of meaningful fundamental advantage to reduced deadliness.