As Yevgeny mentioned, although I would expand:
1. "Anyone threatening its rule" is the key phrase. Please explain how exactly would Navalny, who has no connections in the Kremlin elite, threaten Putin? Either I don't know something about Navalny that you know, or there is literally no threat to speak of. Revolutions aren't made on the street, at least not in the 21st century. Any revolution that has any, even the slimmest, chances of success, must have defectors in the government or the military bureaucracy. In Russia's case, the entirety of the elite hates Navalny more than it could ever imagine hating Putin.
So "The Navalny threat" lives rent-free in the heads of western readers and possibly politicians (although I doubt even that). But you're free to change my mind on this one, if you think you can offer something new.
2. "Persecution" - maybe, as was in the case of Khodorkovsky, who had enough money and connections to turn the entire country on its head. But even he was handled pretty neatly. Now he's an exile funneling millions into supporting Navalny-minded movements and initiatives against Putin. Personally here, I'm wondering why he didn't get a taste of Novichok or Plutonium tea, given he's a far greater and realistic threat compared to nobodies like Navalny.