Answer: The "after [n] day(s)" threshold should read "keep the most recent [n] days of successful samples".

The "after [n] day(s)" calculations for the purge process are based on the most recent successful sample, not the current timestamp. That means the most recent [n] day(s) worth of successful samples will be preserved if sampling is turned off or stops for some other reason. It also means that if there are no successful samples, nothing is purged.