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.
Again, we don't have to speculate. We don't even know who the founder of Bitcoin is and it's still under active development.
In case you're interested, the mailing list is a great way to see what's happening in Bitcoin development: https://lists.linuxfoundation.org/pipermail/bitcoin-dev/
For Ethereum: https://ethereum-magicians.org/
Similarly, even if they used someone else's chain like Ethereum, if the data stored on the chain references something like a CDN endpoint where the real payload is available, the fact that the blockchain itself is still running wouldn't help you much if you didn't have a compatible alternative with a copy of the data.
I am not saying that’s likely today, but rather that it’s easy to over estimate resilience in the case of black swan events.
There's a disincentive for people with mining power to attack the network (assuming they even have enough). It makes their hardware worth less so I can't imagine anyone doing it for kicks.
Agree though that something similar to what you're saying is what Bitcoin hitting $0 would look like. Certainly not impossible but I think it's extremely difficult attack. Even more so now than in 2017.
The real issue for current owners is Crypto seems to have lost it’s grip on the popular imagination. https://trends.google.com/trends/explore?q=bitcoin What happens from here is likely a question of utility rather than speculation.