This is why people are distrustful to voting machines and other techy solutions. Having everything verifiable in public addresses these concerns.
> There is an authority (the government), so the lack of centralized trust of a blockchain adds nothing.
The trust needed is for the government to distribute keys/votes instead of trusting the government to store and count your votes as well.
> a malicious actor will simply spend $20m of compute power to reverse votes or otherwise stall out democracy.
Yes they could. It is however easy to detect.
> so it seems likely if the US uses $20k of computing power to secure the "vote blockchain"
We would of course embed our votes in the much more secure Bitcoin ledger which is much harder to attack and runs without any help from the government (although they could help make it even more secure).