To do the same with 1 vote system, the attacker would need to buy 20x more stake.

Great point! He would not be able to do the attack at all. But with 30 votes he can easily. This allowed Sun to take over the steem - but we're not changing that?

Let's be adults here - this system allows small group of people to control the entire network. 5 users with 10% of stake each can control the network. This would not be possible with 1 vote system. There's a reason why bitshares is dead, steem was controlled by one Chinese and hive is #120 market cap and falling. But OK, let's keep that system and pretend that big investors will come.