The price of cryptocurrencies is determined by supply and demand, just like other commodities. Many alt coins are indeed scams or ponzis. Bitcoin has been demonstrated not to be a ponzi scheme in the classical sense, although it is certainly possible that they end up worthless in the long term anyway.
ok.yes when there is one who is having good demand others also enter to have same and in mean time scammers emerge also
so my question is so mnay saying we use same sturcture as bitcoin or else even more
so why so many in this thread claim that bitcoin only was safe
and there are no others who are genuine like bitcoin
Few other altcoins are exact copies of bitcoin. They all come with variations, different algorithms, premining, etc. In many cases, the creator of the altcoin or early investors have an unfair advantage. Or, even if no scam is built into the coin, the small market cap of the coin allows a single trader to significantly affect the market to their advantage. Furthermore, all of these alternative coins serve little real world purpose that is not already adequately provided by bitcoin, which has much larger market cap and liquidity.
That being said, bitcoin itself is very far from safe. It could succeed in the long term, but it could also turn out that it's nothing but a temporary fad and be gone in a few years. In fact I think the chances are very high that bitcoin will disappear.