I still don't understand why inactive activities that are on the stack don't get reaped, and I'd really love to figure that out.

As of 2025, neither https://stackoverflow.com/a/7576275/1482541 nor official documentation (https://developer.android.com/guide/components/activities/activity-lifecycle#asem) provide correct information because of https://cs.android.com/android/_/android/platform/frameworks/base/+/89ad456ea49cb62615ebdcac83a2515743bbe5fa. Some parts of that fix have changed over time, but with the help of a simple sample application (memory allocations combined with activities spawning) I was able to observe background activities being killed on a range of devices (Android 12 and 15).