An oligarchy is, by definition, self-elected with a lot of opacity.
Inside EOS, the voting process is fully transparent, exactly like for Steemit's Witnesses.
The voting process is permanent, every 120 seconds, the list is refreshed. A block producer miss-behaving with the community will be ejected from the top 21 very quickly by losing all his votes. In Bitcoin, 3 players are controlling more than 50% of the network, same for Ethereum, THAT is oligarchy! In our case, each BP controls 1/21 of the mainchain. Any decision must be the fruit of a consensus from the 21 BP.