The network hash rate is roughly equivalent to 1M Antminer S9 rigs, which would cost something around $2bn, perhaps $3bn.
Your attack scenario assumes an attacker who wants to extract a financial gain from his advantage while keeping a long position.
However, there are scenarios where an attack might benefit from a crashing bitcoin or the ability to deliver a plausible threat to trigger that crash
An example could be a nation state who would like to remind its negotiating partner, how easily they could establish control over a network that holds a substantial amount of foreign wealth. China obviously. Another one would be an investor who wants to short bitcoin in a massive scale (This has been done before, think of George Soro's 10bn GBP bet against the Bank of England)
Crashing seems a lot easier that trying to double spend. A majority hasher could simply ignore all blocks that are generated from the minority and mine only their own blocks, containing nothing but dummy transactions between the attackers wallets. The minority miners would at a 50% chance mine a new "honest" block and at 50% mine an attackers block, giving the attackers blockchain an 75 over 25 advantage. Since there are no real transactions in the attackers blocks, there would be no way to move bitcoins around anymore, holding them literally hostage.
If there is a way to make a profit from a large scale attack, then that large scale attack will be made some day.
Your attack scenario assumes an attacker who wants to extract a financial gain from his advantage while keeping a long position.
However, there are scenarios where an attack might benefit from a crashing bitcoin or the ability to deliver a plausible threat to trigger that crash
An example could be a nation state who would like to remind its negotiating partner, how easily they could establish control over a network that holds a substantial amount of foreign wealth. China obviously. Another one would be an investor who wants to short bitcoin in a massive scale (This has been done before, think of George Soro's 10bn GBP bet against the Bank of England)
Crashing seems a lot easier that trying to double spend. A majority hasher could simply ignore all blocks that are generated from the minority and mine only their own blocks, containing nothing but dummy transactions between the attackers wallets. The minority miners would at a 50% chance mine a new "honest" block and at 50% mine an attackers block, giving the attackers blockchain an 75 over 25 advantage. Since there are no real transactions in the attackers blocks, there would be no way to move bitcoins around anymore, holding them literally hostage.
If there is a way to make a profit from a large scale attack, then that large scale attack will be made some day.