What about just the positive version? Like, instead of being able to downvote somebody for being unreliable, you could maybe upvote people who [i]are[/i]? Instead of having it on a graded scale, it could just be the literal number of people that have gone, "Yep, this person is solid enough to endorse". It's much less outspoken, but does mark out those people who have a positive history of sticking around, without threatening to backfire unfairly on those who do drop out for whatever reason.

It also feels a lot friendlier and supportive, rather than judgemental. Just like we have Likes, Thanks and Laughs, and not "I think this user is a prick" and "Not as funny as the user thought it was".