scheme organizers
Who are these "scheme organizers"? The devs? The core team? The first investors? There is no such thing in Bitcoin.
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scheme's economic activity
The "scheme's economic activity" is exactly what you describe as ...Quote
funds brought by the participants themselves.
... because Bitcoin is most similar to a barter community. It works only if it's used. But it doesn't need "new participants" nor growing value flowing in (as you seem to begin to acknowledge
).Scheme organizers are miners. They issue new bitcoins, but pay nothing to bitcoin holders. In legitimate investments, issuers of an instrument pay its holders monetary or non-monetary value, as I have explained above. In the bitcoin scheme, miners pay nothing to bitcoin holders, which is why these holders can be paid only from funds of other bitcoin investors, like in classical ponzi-like schemes.