I think the answer to "Why do we need to create IndieCity accounts ? and be dependent on their ecosystem ?" is because the alternative is to create your own ecosystem. That means servers and tons of software. The Foundation would have been faced with a huge development task and their focus isn't writing that sort of application. So they picked a partner just as they did to manufacture and distribute the Pi.
I can see why it would have been cool if the Foundation created their own store but I can also see why this would have been impractical if not impossible.
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