Question: Why doesn't the purge process delete very old samples?
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.