To me, this issue has a few things going on. For the people who are afraid of posting LFM's or new to it, you have the unfortunate circumstance of being an unknown in the leadership (loose definition used here) department. Even if you are known in the PUG scene as a solid player, having the star ups the ante of expectation.
And that leads into the second point.
If you are new to "leading" and have a couple of failures, regardless as to whose fault the failure belongs, you having the star means you're the one they're going to remember as leading the "failed" quest next time they're looking at the LFM board.
Lastly, if you are a member of a guild that has had members "lead" failed PUG quests, you may be falling under the "guilt by association" banner when you post your LFMs.
While I have no doubt that there are players leaving the game, I do doubt that the numbers are as such to really have so huge an impact on the PUG scene. I think that it has to do with a combination of people leaving the game, leveling out of range, joining guilds, and joining channels.