People tend to comment what they do not understand. A Ponzi scheme is illegal. Bitcoin has undergone regulations and is accepted by any structure. It is investment and people can earn money with it. With any kind of investments, you can lose your money. It does not mean that all investments are a Ponzi scheme. Moreover, as many have quoted, the blockchain technology has nothing to do with a ponzi scheme. It is very transparent and useful to the society. It is not only about money. It can be used for different social aspects.
Ponzi scheme is illegal only, from when something is defined as ponzi, not before.
But I agree with You, that BTC is not ponzi scheme.
Its a pump&dump scheme on the top of pyramid. This is unusual pyramid, thats why it has not been clearly stated yet. Its a pyramid, where payment for miners in every new generation of coins released is halved. Therefore mining gets 2 times more pointless each time, eventually vanishing into zero.
Trading part is clearly a pump&dump scheme, for undrstanding that, You should read, how p&d work and look closely at BTC price changes. There are certain patterns.
In order to classify BTC correctly one has to have good knowledge in economics, in IT and in math. Few have them. Thats why there are very few to the point critisisms towards BTC.