Pinging Analytics to ask about editor longevity data (:
My understanding is that newbies (<= 10 edits) are more likely to disappear early in their "careers" than they were 5 years ago, but that editors that have been active for years are likely to remain active for years.
It would be interesting, as part of the strategic plan process, to work on improving editor retention. I believe that this may be related to our treatment and training of newcomers (onboarding, civility, NPP, Teahouse, etc.) in addition to external changes in our environment (e.g. the rise of Facebook).