Well, hold up just a bit Jack, the Secretary of State not only using a private unknown (to the public) e-mail for Department business but also keeping her e-mails on an off-site server entirely within her possession is worthy of an investigation. Especially when considering the Clinton Initiative's existence even if they did seemingly appear to properly follow the various loops of the law strongly.
A FBI investigation? Even a Congressional one? Probably not, and also probably not. Maybe an internal one that reports to a Congressional committee for oversight purposes. Then if there were real crimes it could always be kicked over to DoJ and the FBI.
Remember there were a whole host of problems here that were not exclusive to Hillary outside of her possession of the server. Sarah Palin ran into a multitude of issues that was going to drive her out of office and that was petty state government business. I know Colin Powell is a favorite go-to tu quoque but he appears to have been better than Hillary at keeping the private e-mail separate aside from a few slip-ups on rather unimportant matters that the department knew about. It wasn't tens of thousands of e-mails, the entire worth of correspondence of a time in office. Plus, with all the Russia and China stuff, there's the security aspect. (Though ironically, she probably wound up making her e-mails more secure by not using the State Department's system which I assume has long been compromised to hell and back.)
This also was in the most transparent administration ever where this type of behavior was so rampant to avoid FOIA that we wound up with a fake EPA employee earning an award for ethics because the dumbass EPA Director who was using the fake employee's e-mail (again, for department business) forgot to log out and back into her real one.
Hitting Hillary with sending stuff that was back classified is a problem with the entire system of how the government classifies things and arguably the stupidest part of this is that nobody from either party has used the scandal to point out just how insane this system is. Hillary was fine, but there have been lower level staffers who have faced charges for disclosing information that was classified years later or stuff that was public knowledge and then still classified. And the absurd levels of things that are classified by the government would blow most people's minds. That was the entire reason the Republicans knew Hillary had sent something classified somewhere if they could just get the e-mails because it would be borderline impossible not to.
As for its relation to the 2016 campaign, I think it became more noise than anything for most voters. She would have been swamped by the GOP no matter what, this just fell into their laps. The FBI investigation is the part that kicked it up a notch but I'm unsure as to it being defining enough to flip voters to Trump. Though it may have hurt turnout I don't know how much that can be measured against Berniebros who would never be converted or her inability to turn certain cleavages like Obama.
