Its not true at all the everyone cheats (I have not yet done so, so I know there is at least one person, and I am entering my upper years in the fall). I would guess that cheating rates also depend on what major you are in. Cheating on a math exam (my major) is damn near impossible unless you are literally copying answers (and how tests are set up makes that difficult). I expect your statement holds true for the non-academic (and frankly less rigorous) areas like business, marketing, etc. But if you don't know what you are doing on a math exam, no amount of past plagiarizing will help you, as it will be painfully obvious you have no clue what is going on. And using ADHD medication to stay focused is not cheating, it is dangerous, but it is not cheating (and it is unlikely to actually help you anyway).
But on the issue at hand. Harvard, more then any other Ivy league school, is criticized for being a degree factory, rather then an institution of learning. Although I would guess that Mcnally and others will be allowed to return, it is fully within the realm of possibility that they will not allow them to come back at all, to make an example of them. This is especially the case if none of the cheaters were kin of large donors. Do not underestimate Harvard's vanity, if this story keeps rolling along (unlikely as that may be) you could see long term consequences for this group.
From what I understand, the cheaters took a take home exam and used other resources to complete it (but it was short of plagiarism if I am not mistaken). I have never had a take home exam, but on the face of it they seem ridiculous. I would be inclined to change the policy so that take home exams were "open book", and if professors disliked that, then they must hold a controlled environment exam.