I honestly, real talk, don't think this matters at all except politically, so my response to it is exclusively political.
Who cares if the Secretary of State has a private personal email server? She still needs to send public emails to actually get anything done, because those emails need to go to the State Department.
Moreover, frankly, I don't believe that no other politicians have private personal email servers, for the simple reason that everybody has private personal email servers. I have one! It's not in my house, but that's mainly because I'm too lazy!
But even if we pretend that the SoS has never before had a personal email server, they've still had personal conversations, personal phones, personal secretaries and messages, and generally personal lives. It's both necessary as a human and inevitable as a Secretary of State to have private communication pipelines.
Can you explain what you think people would be saying if this were Tom Cotton, or what you think they should be saying about Hillary? Because I honestly don't get the problem.