I'd go for a combination of (1) and (4). Not so much a 51% attack as implicit agreements between dominant miners making transaction fees high as the block-reward halves. Meanwhile competitor PoS coins offer very low fees because they don't have to pay for monster hashing rigs. High fees discourage Bitcoin use and make users flee to other non-PoW coins. Fewer transactions further reduce the income to miners. Lack of demand reduces the BTC price. It's a downward spiral. Miners give up, the network hash power drops, and the network becomes less secure, making users even less want to trust it. Eventually someone mounts a proper 51% for laughs and to be able to say that they were the ones who destroyed Bitcoin. The end.
Very interesting thoughts, thanks for taking the time to express. Definitely feasible I think, especially when combined with "People not spending BTC" and the resultant apathy/loss of interest exaserbated by a falling price.
I'm not sure if what we are seeing now is the start of this. The block reward remains high, and fees are relatively low. However, the high block reward means that Bitcoin inflation is running at around 10%/year. That's part of what is depressing the price: all those miners selling. It makes Bitcoin a terrible store of value - you'd be better off holding dollars (2% inflation). Perhaps more importantly, the writing is on the wall. Despite being ridiculed, PoS have been running for a long time now with no core security issues, so they have proved themselves a viable alternative. Anyone doing due diligence into the state of crypto-currencies today will realise the above scenario is at least possible, if not likely. Perhaps that's why no-one is rescuing the BTC price.
Interesting article here on PoS system downsides.
"However, proof of stake, as implemented in nearly every currency so far, has one fundamental flaw: as one prominent Bitcoin developer put it, “there’s nothing at stake”."
https://blog.ethereum.org/2014/01/15/slasher-a-punitive-proof-of-stake-algorithm/Must admit I didn't understand all of it but!!!! I get the general gist I think. Do you think Ethereum could be the PoS system that suceeds?