Can you spell out the specific attack that you think could take place? In practice, I don't think a large scale double spend would work.
Someone with 51% of the network (as you say, costing $100M+ of mining equipment currently) could execute an arbitrary double spend attack. If they are financially motivated, they might try an attack like this:
1) Buy a large asset (say a publicly traded company, a huge tract of real estate, a closet full of gold) with bitcoin.
2) Wait for the transaction to clear. For a large asset purchase such as the above, this could take weeks.
3) Once the asset is in hand, use your mining power to reverse the transaction.
There is a critical flaw in this plan: you are obviously stealing, and the justice system will treat your actions as theft. If you try this for a reversible asset transfer, like real estate, they will reverse the transfer in addition to punitive measures. For gold, you might be able to hide the gold somewhere, but again at that point it is just common theft.
Attacks that are not financially motivated are a different story; a nation state or other party could try to 51% the network simply to destroy bitcoin. Of note here is the fact that nation states have more straightforward ways of hurting bitcoin, starting with outright bans.
I am not thinking about attacks for financial gains - if you already have 100 million Dollars there are probably better ways to get some more money than trying to double-spend Bitcoins. The really bad scenarios are probably adversary states or terroristic organization shutting down the circulation of money in case a state or a couple of states decide to use Bitcoin as the primary currency.
Someone with 51% of the network (as you say, costing $100M+ of mining equipment currently) could execute an arbitrary double spend attack. If they are financially motivated, they might try an attack like this:
1) Buy a large asset (say a publicly traded company, a huge tract of real estate, a closet full of gold) with bitcoin. 2) Wait for the transaction to clear. For a large asset purchase such as the above, this could take weeks. 3) Once the asset is in hand, use your mining power to reverse the transaction.
There is a critical flaw in this plan: you are obviously stealing, and the justice system will treat your actions as theft. If you try this for a reversible asset transfer, like real estate, they will reverse the transfer in addition to punitive measures. For gold, you might be able to hide the gold somewhere, but again at that point it is just common theft.
Attacks that are not financially motivated are a different story; a nation state or other party could try to 51% the network simply to destroy bitcoin. Of note here is the fact that nation states have more straightforward ways of hurting bitcoin, starting with outright bans.