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At least, they can stop or limit payments for introductions.

Thats the beauty of Blockchain technology you cant stop it, its decentralized. Read about Bitcoin, very similar technology.

They can. The decentralization is illusion when over 51 % belong to a group who can easily collude. Look at what happened to Ethereum.

what happen to Ethereum? its still running, just 2 of them now. People didnt want to fork and no one can stop them from using the older one.

Yes, but ETC is way cheaper then ETH. Essentially, ETH creators broke all the trust because of money.
The same is here. Everything is essentially controlled by this website - Steemit. Very few people will bother reading the blockchain. And hard forks are done regularly and easily.