So, could the Ethereum community get together and agree to rewrite the blockchain and undo this transaction? Perhaps they could vote on it and have a hearing of the facts. Of course that introduces its own tyranny but is it possible?
It is possible. That's why you have Ethereum and Ethereum Classic, two different chains. The latter one is the unaltered chain, while Ethereum (which is the most popular that's everybody are using) has been forked in such a way that you're describing once after a large hack.
Specifically, it forked because even though the system is specifically designed to make rewriting history impossible, the hackers screwed so many users that they decided to undo it by ignoring history and starting from before the hack.
... Lest anyone ever think Blockchain tech is somehow immune to network effects and social considerations.
Technically, rather than ignoring history and starting from before the hack, they added a nonstandard transaction (not generally allowed) which reversed the effects of the hack.
This did not revert other transactions that happened after the hack.
But, yes, blockchain stuff is fundamentally based on consensus about what the rules are, and people/organizations with more social influence can [...] .
The chain is by nature append only, so you'd have to fork it, which they sure as hell are not going to do for a "little guy," to put it mildly. At least, that's my layman's understanding.
