You're correct. Miners don't control security of the blockchain. That's the purpose of witnesses.
About the potential risk, have a look at @dan's response in my previous post
Having Steem mining queue controlled by one miner is not really a risk like in other crypto-currencies.
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Excellent reply! Thanks for pointing it out to me. That confirms what I was thinking. The additional witnesses and backup witnesses keeps things nicely distributed.
dominating the mining queue is like being a top19 witness in terms of money.