Harry could destroy his own reputation in order to save Hermione, by (for example) threatening to forever abandon Wizarding Britain.

Shouldn't they take that for granted already? I mean obviously he's going to have absolutely no remaining loyalty to the state - or at least the power structure - that did that to him. They should all expect to die whenever Harry finds it convenient to overthrow them. Or is that just what I would do?

(Any sane politician who was planning to make that sort of move against a potential emergent power like Harry would also see to it that they were killed, crippled or framed as a matter of course. You don't go around recklessly making enemies and leaving them free to gather power.)