You're also conflating the responsibilities of State Department personnel with regard to information classified by the government they've sworn to serve, and the responsibilities of other nations' diplomatic personnel with regard to information originating in the government of a state foreign to them.
Neither seems especially conducive to a useful discussion of the matter at hand.
More to the point, shadow IT exists for a reason. Taken out of the context of the State Department and the political sphere, this was classic shadow IT. I've used shadow IT, and I've provided shadow IT, because I've worked a lot in large, sluggish bureaucracies, and that's How Things Get Done sometimes. "Security" becomes a catch-all excuse for laziness and cowardice.
If she felt like she could do her job with the existing State Department tools, she wouldn't have set up a shadow IT operation, period. It's not like she's completely ignorant of either operational security or political ramifications. To do this, she must have felt thoroughly hampered by the existing system.
Sticking around at the expense of her oath of office doesn't seem to have worked out all that poorly for her, since she's still apparently a serious contender for the presidency. Should she end up in it, one hopes she'll take that oath a little more seriously than she did the last one.
I'd say she won.
Moreover, the security of individual emails depends on the security of the recipient as well as that of the sender. Sensitive/classified emails sent to officials of non-US governments are subject to whatever security they might have. The only solution to this leak vector is to completely ban email as a means of communication - which gets right back to the core requirement that the Secretary of State must be able to communicate quickly and efficiently.
I'm not arguing to "throw security to the winds", and I don't think that's what was done here. Again, I'm asserting there's no reason to believe an email server administrated by the State Department would be any more secure than an email server administrated by skilled private admins.