If you are a particularly proactive community manager, you might notice when a member starts to become inactive and check in with them to make sure everything is OK or to see if there is anything you can do. In demonstrating that you care, you might be able to bring them back into the fold.
Why isn’t community software helping us do this?
I envision this working as follows. The person managing the community can define scenarios that would demonstrate a meaningful change in activity. For example, if a member has not made a post this week, but had made at least 30 posts in the 30 days before that, I want to see a flag in my dashboard.