In terms of value stored my wild speculation is that whatever shady business is going on with tether will eventually implode and the contagion will be extreme.

In terms of value stored my wild speculation is that whatever shady business is going on with tether will eventually implode and the contagion will be extreme. People have been saying this for 10 years and nothing has happened yet so who knows. But in terms of the network itself failing there is really not much you can do. Fight club / Mr robot style terrorist attacks are pretty much out. Maybe a major nation state would have some success but I doubt it. States might have success sowing confusion by compromising core developers and pushing software updates to nodes that do damaging things. I don't see this totally killing the network though. Banning through legislation needs to be done across all or large majority of nation states which I don't see as being realistic. Though maybe as the environment faces increasing catastrophe this will become more possible. Quantum computer attacks can be defended by relatively trivial forks that everyone involved has a reason to support. I think you can even do Lamport signature stuff today with the pay-to-script-hash stack machine language. Maybe someone finds an exploit in sha2 and launches a surprise attack. That would cause significant damage. I would guess the network would fork itself onto a secure hash function quickly enough. If someone takes out the internet for good that would probably kill the network. In all of these scenarios there is the possibility of restart if the conditions killing the network revert.