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This is easy, you can judge them by the results they are achieving. If more and more people are being drawn to the platform, then they are doing a good job of curating the content and inspiring people to create good content. If the growth stops or goes into decline they are doing a poor job. Simple.

That does not tell me it is "optimal".

I thought the entire point was to eliminate gatekeepers.

Well I think the only way to eliminate the gatekeepers is to grow the network and further distribute the steem power across a wider network, where eventually there is so many dolphins that the whales become almost irrelevant.