I'm surprised that you seem to think that "slacker"/"non-slacker" is an intrinsic immutable property of a person rather than a situational response to incentives. I think most critics of tenure would argue that granting life long job security can and does cause "non-slackers" to adopt "slacker" behavior since they no longer have to compete for their job. The fact that someone worked hard to get tenure does not imply that they will continue to be productive when they have no competition.