I don't think it's so much "link churn" as looking at the number, quality and time of appearance of links as they relate to the inception date of the domain in question.
We do believe in this, but not in so far as it ultimately results in a PR of Zero, (which is a rather dramatic event in itself). It would appear to us, that a lack of new inbound links, these days results in a continual decrease in PR, (or PR as displayed in the tool bar).
Perhaps there is cause to think about how pure age of the site is being factored in, (which used to be quite significant) and the time line of links acquired. Old ones were more valued than newer ones, but now newer ones seem to put the luster on the apple.