I don't think this email scandal is that big of a deal in terms of Hilary Clinton's presidential campaign. The rules that would have required her to use only a government email address for her work-related emails didn't go into effect until after stopped being Secretary of State. So she did not violate any rules, although you could try to use the overall sense that she was trying to protect the privacy of her emails into some sinister cover-up if you are so inclined. Plenty of people are so-inclined, and that is what we are seeing now. But it's early. The American public has a remarkably short attention span. To the extent the general public will even care, this is going to seem like ancient history by November 2016.
But what this non-scandal does illustrate is the huge downside of having a presumptive presidential nominee this early. If something more serious were to emerge, something that really did pose a threat to her viability as a candidate, it's not clear what the Democrats would be able to do. At this point, there is no clearly viable alternative presidential candidate to Clinton. It is much better to have a competitive primary season than to have one candidate who no one else has a plausible path to beat (absent a viability-challenging scandal for the front runner, of course).
But what this non-scandal does illustrate is the huge downside of having a presumptive presidential nominee this early. If something more serious were to emerge, something that really did pose a threat to her viability as a candidate, it's not clear what the Democrats would be able to do. At this point, there is no clearly viable alternative presidential candidate to Clinton. It is much better to have a competitive primary season than to have one candidate who no one else has a plausible path to beat (absent a viability-challenging scandal for the front runner, of course).