> A public blockchain is a mere write only database
Precisely. It does nothing to address who can write to it. Ideally, it should be people who are eligible to vote (citizens of the country that are over 18 years old). That's a difficult problem to solve even with paper ballots (especially in a country that doesn't have national IDs), and pretty much impossible to solve with any sort of a digital solution.
> ...that everyone can see.
...or nobody can, as you'd see by reading the article that was submitted.
> Every transaction is available to 7 billion people.
7 billion people can see how to vote as well. You're not asking 7 billion people to vote, but a small subset of 7 billion people.
Every transaction is available to 7 billion people.
Vs paper voting which is only as legitimate as every voting station.