Old coins are getting piled up since upcoming projects and updates of newer coins are continuous and enormous, right now the market is driven on a coins' popularity to investors and not its potential, of course buying old coins would still get you some profit but you cannot expect the same profit you'll get from hyped and famous coins.
Primecoin is one of its kind with a PoW algorithm that contributes towards the discovery of prime numbers. It's a pity other crypto projects haven't copied its model.
This is actually so true, but if I had a chance I would rather mine than trade old coins that as good as Primecoin.