I could start up a "Casascius Block Registry" by myself, which would be some RPC web service where I sign all the blocks that I think are good, and allow people to query me for my signature for any given block. No one has to care, nor does anyone have to pay attention to what I'm doing, and more than likely, I get very few hits from only a few curious geeks.
A few other individuals who think alike might do the same thing. So after all, you have a bunch of obscure geeks signing blocks, nobody cares about these signatures, nobody queries them. Consider it a form of digital masturbation. Maybe a dozen people do this.
Then along comes an attacker. Panic ensues, along with widespread disagreement as to what to do next.
I don't like it. I hope you read on as to why ...
The dependency on individuals makes the network weaker not stronger. Say Bitcoin someday did process billions of dollars. The potential for large scale fraud will attract organized crime. Coercion, bribery, or outright assault of registry operators isn't a far fetched idea. Organize d crime uses violence and intimidation to orchestrate much smaller crimes today.
If a family member ends up kidnapped are you going to put the network over their life? Or will you "approve" and sign the malicious blocks and anything else they "ask" you to do to ensure your loved ones aren't harmed? Alternatively if the network ever did grow dependent on your service expect leverage and pressure from a government. For example they would want backdoors so they can flag transactions they deem as illegitimate as not valid.
I think a "proof of stake" or "proof of history" is a potential mechanism but it needs to be more protocol based where those w/ stake or history are chosen by the protocol and the number of entities needs to be much larger not a handful but maybe a couple hundred. Granted a couple hundred nodes having a more important role is "more centralized" but still sufficiently decentralized to make attacking an individual ineffective.