Actually ich does not die out. That was based on one paper. Seems to have not been replicated. It may have been a particular strain. There is much more experience with ich lasting years.
link
Here's the thread where the ich line dying out is discussed.
But Steven Pro later posts about this paper disputing the original researchers claim. Here the strain lived for over 2 years.
Yoshinaga, T. & H.W. Dickerson. 1994. "Laboratory Propagation of Cryptocaryon irritans on a Saltwater-Adapted Poecilia Hybrid, the Black Molly" Journal of Aquatic Animal Health 6:197-201, 1994.
Paul B. had ich live in his tank about 30 yrs. It can go subclinical. You can live with ich for years. But if you have a stress event like temp spike, tank move you will very likely get a massive ich outbreak.
The fact that ich falls off at nght is actually rather insidious. Fish tend to sleep in the same spot each night. It means when the theronts hatch the fish is likely to be right there sleeping to get re-infected.