I would argue that they’re already quite affordable in a marginal sense — the marginal cost in power to supply 51% of the hash rate for long enough to attack th

I would argue that they’re already quite affordable in a marginal sense — the marginal cost in power to supply 51% of the hash rate for long enough to attack the network is not terribly high. These attacks are not affordable in a capital sense when the miners are running near capacity. There isn’t an AWS with unlimited piles of hashing ASICS that you can rent for the hour at a reasonable price. You need to buy or lease these things, Ave capacity is finite. But I suspect that, if mining profitability per hash drops enough, then the capital limitation on attacks will go away.