> The moment the transaction fees and block rewards go down, the very expensive Prof of Work mechanism will weaken and future transactions are unprotected. Exa

> The moment the transaction fees and block rewards go down, the very expensive Prof of Work mechanism will weaken and future transactions are unprotected. Exactly. One critical, related vulnerability that I haven't seen people discuss: Right now, every new block gives miners about (12.5 new BTC + 3.5 BTC in fees) * $15,000 = $240,000 So miners are making a quarter million every ten minutes. That incentivizes massive capital expenditure on mining hardware and massive electricity burn. If the price drops significantly -- or Lightning Network etc solve the scale challenges, and transaction fees come down -- or in June 2020, when the block reward halves -- any combination of those may lead to a future where it's uneconomical to mine again, except in places where electricity is unnaturally cheap. When that happens, tons of mining equipment will fall idle. The network difficulty will adjust down, and in the immediate term Bitcoin will continue as normal. But now there's a big pool of latent hardware left over from the bubble, which can be activated at any time. This may break assumptions about the cost of attack. If, say, a Chinese mining group owns tons of hardware that is no longer economical to operate continuously, there's nothing stopping them from shorting BTC and then doing a "spawn camp" repeated 51% attack. (If you already own enough hardware, then such attacks are pretty cheap--you only have to burn tons of electricity a few times, for a few minutes each time.)