The Thing Missing in Scrum

I am working with Scrum very successfully for several years now. And it is by far the best methodology crossing my way in software development. Yet, I think there is one thing, missing in Scrum - its slack time.

Most Scrum coaches would probably say: "No, no, no... Slack is not missing. It is ...
  1. ... automatically emerging, because the team is self-organized. It is in the responsibility of the team to take whatever slack is needed to accomplish their work."  
  2. ... an optional add-on to Scrum. You can just implement some slack additionally as you like. But Scrum does not lack slack. Slack is not missing more in Scrum than it is in traditional work environments."
I personally think, those arguments are not valid. 

Argument 1 is questionable: Nobody knows when he "needs" some slack, to be creative and thoughtful. Slack happens to help sometimes and does not help on other occasions. Thus, slack must just be there from time to time (for every single individual - not the team as a whole), so you get a chance to let your brain defocus and accidentially stumble over some very good ideas and thoughts. The tricky thing about slack is, that you will not "miss" it at a specific point. The need for slack ist not foreseeable. Thus a even a self-organized team cannot be expected to know when to take some slack time.

Argument 2 has its flaw, too: You could similarly say that Scrum has no need to contain the role of a Product Owner, but one should implement it as an add-on to Scrum. But this is obviously wrong, since you need somebody giving input to the team, having a more global vision, and beeing responsible for creating business value. Scrum needs to define this role, since it removes the role of the project manager (who was responsible for this tasks earlier).
I think the same is true for slack: Scrum is much about focus and productivity. This is not wrong, but it lacks a necessary concept of slack. Even more, it squeezes some natural amount of - often healthy - slack out of the team that occurs in most classically organized companies from time to time. The point is: Because you are working so much more focused on creating value for the customer (which naturally comes with Scrum) and achieving Sprint goals, a certain amount of slack should be built in, too. This is why I personally think that slack is the missing thing in Scrum.

I guess it would be of great value to Scrum if the framework contained some slack space for every individual in the team (e.g. everybody has one "free" day, where he is not expected to contribute explicitly to the sprint goals).

What do you think? Is there the necessity for slack time in Scrum or are there indeed mechanisms in place substituting this need? What are good mechanisms to build slack into a Scrum team?