[giaour]

> PoW security is a vector, and doesn't flip from secure to insecure at some specific point.

The network does flip from secure to insecure whenever the cost of mounting a 51% attack dips below the amount an attacker could expect to gain. Markov and Schoar [0] made a convincing case last year that as minted rewards from mining drop, the active mining pool will naturally shrink and become more and more vulnerable. PoW does not prevent misbehavior; it establishes an incentive system to encourage participants to behave properly, and that incentive is subject to realignment by market forces.

[0]: https://www.nber.org/papers/w29396

[danuker]

It's not a network-wide flip. Certain victims suffer only, and they are very few - the ones not waiting for enough confirmations after receiving a large amount. For example, right now with a cost of mining of $1M/h, after receiving $2M, you should wait for 2 hours (12 confirmations), before accepting it as settled.