That's a distraction from the real problem: what value does someone get for keeping a blockchain running? If the founders of the project have walked away from it, how many volunteers are going to care to support an orphaned system at all rather than putting their resources into something they can get paid more for?

The usual answer would be to keep a pet project alive but that runs into the realities of blockchains: the technology is inefficient by design and that means that most of the real work and data happens off chain. Having a copy of the chain itself is likely of minimal value if the company hosting referenced data stops paying those bills, an oracle is no longer available, the client application stops being developed or is not released by its owner, etc.

This means that you can't say anything about the long-term survivability of the chain itself without saying what it's used for because that'll tell you both how many people care about it at all and how committed they'll be to continuing to maintain the chain and its clients.