I'm wondering whenever it's worth buying and holding old coins for the long term. 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. Your input will be greatly appreciated. Thank you. 
Cryptocurrency is evolving, coins come and go even the new coins that you are seeing now, have a possibility to cease, there are old coins worth investing, Bitcoin and Dogecoin is the trending coin this year and they are old coins, so we can safely say that it's still worth investing in old coins, your portfolio should consist of old and new coins, there are old coins that also evolve to keep up the present need of the community.