Actually, no. You need more positive contributors than
negative
contributors.
Giving the positive contributors more power makes their work easier, but
it's not necessary.
I disagree. Vandalising is easier than cleaning up vandalism, so
without the ability to block vandals, we would need far more
vandal-fighters than there are vandals, and with more vandal-fighters
you get diminishing returns. I don't think Wikipedia could exist these
days without blocks, and all these proposals basically remove blocks.