I think what you are describing in your opening post has one flaw: You describe an asset that is sold to people for their fiat money with no real value behind it in return. This for Bitcoin ist not true, since bitcoins were not sold to people by Satoshi Nakamoto. He only created the network and let people do work (mine) to get the bitcoin. If satoshi would have just sold all coins to people and then left it would be a scam, I can agree with you. This is done by many premined coins but as I said was not done with bitcoin. So in order to get bitcoins you also have to perform work, which makes it in return also debt based. TADA: it is not that different form other currencies, except it is digital.
It's a scam in a sense that Satoshi in the Whitepaper claimed that his system produces money. Money is an economic resource from which people can benefit without new buyers or investors. Also, no one knows how many addresses Satoshi has in the system. So he is essentially benefiting from the disinformation written in the Whitepaper. Ho got real economic resources and left the holders only with units of his imagined number (21 million). Which is why all these holders need new buyers or investors to get the resources back. Hence, a pure investment scam.