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