Since there is a limited supply of Bitcoin, people are buying Bitcoin now with the hope that there will be more demand for the limited supply in the future. There is a hope that Bitcoin becomes "useful", thus driving demand. Since supply is fixed, demand causes an increase in value.

So, it's a speculative "investment". I suppose whether it's an "investment" depends on your definition of "investment".

It's useful and it has demand for its use. But most people buy it only to make a fortune. This makes it a Ponzi scheme. No contradiction here.

A Ponzi Scheme relies on a central authority, BTC has none. I think you just mean "scheme?"

There is Satoshi's wallet in the root, no need for authority as we can see.

Yes, there is need for a central authority, Charles Ponzi for example. They actively and consciously channel the funds from new, defrauded investors, to that authority.

BTC is not a Ponzi Scheme ; Satoshi is not steering the profits from a pump-and-dump. Passively profiting off new investors buying BTC does not make a Ponzi Scheme.

Market manipulation is not a Ponzi Scheme. A Pyramid Scheme is not a Ponzi Scheme. "Ponzi Scheme" has a real definition.

Agree, I've used it in a more general sense. I mean it's just like a pyramid.

It's not a Pyramid Scheme either, there is no central authority. The benefits of a Pyramid scheme are specifically not investments, but instead explicitly payments for recruiting. Pyramid Scheme also has a real meaning.