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How can we be sure that Steem creators and steemit.com owners will not promote their own interests?

Because the system would have to be rigged in the code...which is freely available to anyone to audit or investigate on github:
https://github.com/steemit/steem

This makes sure that there is the ability to check that the software is in control of the system.

An issue can and will exist if steem becomes immensely popular and whales can control what posts make it to trending.