It's not quite a simple issue. I think we know vandalism is a "long tail" phenomenon, i.e. statistics of average reversion time get dominated by some very long-lasting bad edits. So for example median and mean reversion times may be very different. The question is whether one reads that as "soft security" actually breaking down, or as the comment that flagged doodads are a bit late to the party or will effect a big improvement. And then there is the issue of people not blanking their watchlists if they stop using them, which would conceal the effectively unwatched pages from the admins who would otherwise watchlist them. The latter problem could be addressed by database searches looking at watchlists of those who edit little.
Charles