Sounds like an oligarchy rather than a decentralized system.

You would think so but in fact it's proveably more decentralized than any current crypto.

The arms race in crypto mining has led to an impossibility of solo-mining. Miners now join a pool or will never see a dime for their efforts. Overtime smaller pools dwindle and merge or die out until only a few survive.

Believe it or not bitcoin has less than 10 pools controlling the network - which is highly concerning due to a possibility of a 51% attack and ethereum is no different.

The difference is that there is that anyone can create a pool without permission from the other pools. There are subreddits with more than 21 mods that have been easily infiltrated and taken over. You can't socially engineer a mining pool

While I appreciate your questions I get the feeling that your trying to poke holes in a very well researched and funded project based on what you read of it in my comments.

There's much more to the project and a lot of brilliant people with over a billion dollars in funding making sure this thing is bulletproof. So if you're interested in learning more about it I'd recommend you read their whitepaper as it will be a much better source of information.

However to quickly answer your comment, the 21 are elected by vote and are easily replaced by vote from the users they serve.