Fully decentralized blockchains are kept alive by full nodes that validate and distribute the current ledger to new users. This means that the network will function as long as there are nodes left and miners adding new blocks. Storing an independent copy of the ledger on all the full nodes is one of the ways that a blockchain maintains security. It removes the possibility of a central server going down to deny service. The many redundant copies also mean that long after a project is no longer popular it is extremely likely that copies of the full blockchain exist on forgotten disks. The network doesn’t truly die until every last copy is erased as it could be brought back up in a few seconds by a single node.
Users might adopt old blockchains for other purposes once the dev team gives up supporting it. Using an existing chain doesn’t really provide any value over forking the source for the original and customizing it, but the fact that it can be done creates the possibility of a “Ghost Chain”- a blockchain that is no longer being developed by a team for its purpose and instead is kept alive by a few users. I’m not talking about a hard fork, as that requires a sizeable community to keep the fork relevant.
A notable case study of a chain dying and coming back to life is Eden. It was a classic masternode scam in which users buy a large amount of coin under the premise that in the future those coins will grant them the rights to some of the newly issued coins distributed by the consensus algorithm. It took place in April after the second big price dropoff of 2018, and the project ran away with ~100k USD in four days. The developer deleted the github, but copies were kept by community members who re-grouped and kept the project going with a new direction. In a moment of poetic justice, the scorned eden community chose to focus on accountability in ICOs, and re-launched as Reden.
It is best said in their whitepaper
The community voted that Reden’s new purpose should be “De-Risking Alt-Coin”, providing services to making it harder for scammers and unprofessional teams to undermine the enormous opportunity alt-coins offer genuine development teams, and their investors.
The admirable effort is still in early stages, but it illustrates the value of a strong community. A coin that only existed for four days and had its source code purged still refused to die. As time ticks on, increasing numbers of blockchains will fall by the wayside to accommodate newer projects, but these wisps of data will be floating around long after their creators are gone.