this is just my opinion....that most old coins are not worth for the long term as they depreciate in price over time. Only Bitcoin and Ethereum are the exception, since they're still one of the most popular coins in the mainstream world. It's best to "hodl" these coins alongside new ones with active development and innovation, in order to prevent many undesired losses in the future.
Right now, there are many old coins that are still alive. But they're often "under the radar" as people are mostly focused on profits rather than utility. There are many good coins like Namecoin and Primecoin, each with their unique features in the crypto/Blockchain space. AFAIK, there are no other coins like Primecoin with an innovative PoW consensus algorithm based on prime numbers. It helps our society with the discovery of prime numbers, putting PoW's "energy consumption" to good use. Sometimes I wish a new dev team or the community would take over the development of Primecoin, in order to further improve its consensus algorithm. Being one-of-a-kind coin, I consider it to be a hidden gem. Namecoin, on the other hand, is a good old coin with a decentralized domain name system. It was the first cryptocurrency to ever make use of a decentralized DNS. Ethereum may have taken the lead in decentralized domain names, but Namecoin is still an alternative worth considering.
No matter which old coins you choose to "hodl" for the long term, it's important to never put all your life savings on them. The risks with old coins are much higher than shiny, new ones on the market. You could end up "holding the bag" someday if your old coin of choice becomes abandoned in development. This in effect, leads to exchanges de-listing the coin, putting trading activity all the way down the drain. You can still use the Blockchain if there is at least 1 node and miner on the network, though. I'm still "hodling" Spreadcoin, despite not having any exchanges available. The Blockchain is still alive and running, which makes the cryptocurrency useful. Let's hope that abandoned coins like Spreadcoin experience revived interest from a new dev team sometime in the future.
There are many factors and reasons that result in project failure, closure or simply abandoning from the team or developers. Some of the main factors include:
Outdated business model.
Team disputes or team losing interest.
Use case not popular.
Bigger and better product in the market already available with no space for more projects.
Failure to attract large userbase or community.
Failure of team to follow roadmap and delievering on the promises multiple times.
Investors losing confidence.