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.
In your system, what prevents people from just making multiple accounts and spreading out their stake?
Nothing but it's much more expensive. You need 20 accounts that have more stake than the rest of the community instead of just 1.
If I have 100 mln HP I can choose all block producers that I control - and they do what I want, for example nullify your account.
With "my system" I can choose only 1 block producer - nothing changes. I can also split my my stake between 20 accounts (5 mln HP each) but still I don't have majority to put on top block producers that I control.
Basically the current system allows small group of people to control the whole network - this is why no big investor even looks at Hive and why we will keep declining with occasional pumps.