I have played with this a bit lately and my conclusion thus far is: The idea of trying to bind everything to a single private key is such a bad idea for the average person. In order to truly secure a private key you have to go to pretty extraordinary lengths. It is not easy. It is not, "common sense."

Like most of crypto, the basic immutable nature of things is simply bad for humans. Here, your private key is eventually going to get stolen because you have to type in your private key for every login. It creates a phishing/key-logging jackpot. And once the attacker gets you private key there is no recourse. No password reset. No way to regain access. Your accounts are forever compromised. This is the problem with "decentralization" in general. All of the benefits it brings are completely washed away by the mundane daily activities of being human.