They are also a good example of positive middleman behaviour. While they take their cut, the value they provide to both sides is huge.

They are also in a position where they are still easily replaceable. Their dominance is from doing it well, not because they have an absolute lock in.

Part of why this works is because they don't have to prioritise short term profit over long term. Most companies like this get brought up and pumped dry. Valve seems to be the exception.