[saalweachter]

The problem with shedding hardware from the Bitcoin network is that it undermines the core of Bitcoin's security.

Let's imagine that 2025, and we hit the fourth mining-rewards-halving. It's now unprofitable to mine given the existing numbers of miners, and users refuse to accept higher transaction fees, so 50% of miners exit, leaving half the profits to half the miners.

This means that 50% of the Bitcoin hardware is now on the market, being sold on the cheap. What do you need to compromise the security of the Bitcoin network? Why, 51% of the hardware--

[maxerickson]

I think the practicality of the attack will still depend more on the price of bitcoin at that point in time than it will on the sudden availability of all the least efficient mining hardware.

At some point obsolete ASIC rigs should be coming on the market for cheap (but right now the reward isn't anywhere near 'full' in terms of the amount of electricity apparently being used to chase it).

(FWIW, I expect a crisis of confidence long before 2025. Let's see how many miners are left after that happens.)