I don't think the churn is a big problem yet, but I think the position of stories on the front page uses a formula similar to gravity. Upward acceleration is determined by upvotes/time and downward acceleration is determined by the time since submission. pg could always decrease the acceleration of "gravity" on the formula (to allow popular stories to stay on the front page longer) and/or decrease the "acceleration" per upvote (to prevent less popular stories from jumping to the front page so quickly).

So if this becomes a big problem, it shouldn't be too hard to fix.