Firstly, if this isn't clear not all altcoins are shitcoins. In my opinion, shitcoins are any crypto that serves no real purpose, take meme-coins for example. And even these shitcoins are not exactly Ponzi schemes by defination, as others have mentioned since they don't promise any returns, but some shitcoins do work like Ponzi schemes.
Take these recent Shib, Squid Game, even Doge examples, early investors who were sensible enough to get out at the peak (just like Ponzi investors) made some insane profits but others who bought near the top are left with holding their bags or cuttings massive losses.