I fully agree with you on this! No private libraries! If the writer is being subbed, they ought to know.
Now that that's settled, I'm looking at two libraries. One is for the stories a reader is actively following. This library is what determines the story's rankings.
Now, say, the reader doesn't interact for 90 days. Then, the story will automatically shift to their second library and the the sub will not affect the story's rankings. If you think about it this way, it shows that the reason why the story isn't growing is because the reader, for any personal reason, is not able to commit to the work. It's not that the writer's story is trashy.
So, this way, three things are fulfilled:
1. Writer comes to know the reader has subbed.
2. Reader engagement works but only until a limited period of reader's inactivity.
3. Once reader is inactive, the story shifts to their read-later library and the reader doesn't lose the story. Plus, that reader no longer contributes to the ranking of the writer, unless they resume engagement.