That's the thing, the top 17 witnesses hold absolute control on the blockchain. Justin Sun used exchanges' custodial money to vote up 20 of his witnesses up to the top and made decisions that could destroy the whole ecosystem. This proves that he could do even more centralised decisions all by himself, and if he wanted, he could basically shut down any account, topic, tag, keyword or anything he wanted. The moment when Justin sun controlled the top 17 witnesses, this blockchain was literally owned by him. Luckily this is not the case anymore
But if he made an effort, he could revert the situation in an instant and our witnesses would have to create a parallel Steem blockchain (a sister fork) in order to keep running it out of Justin's control.