>It's weird how so many of the arguments against what I've originally written pretend that it matters whether the reward comes from a block reward or transactions fees. It does not matter.

It does matter. Block rewards are a transfer of value from bitcoin owners, which increases as people BUY bitcoin. Block fees increase as people USE bitcoin. Those are two separate things. What we are seeing now is mining that goes far beyond the mining necessary to secure the network, and that's due to (temporary?) speculative demand which increases the value of the block reward.

>And the fundamental idea behind proof-of-work that they amount of work you're "proving" must be enough to make double-spend attacks infeasible.

well, infeasible by a single antagonistic party. There's also the cost associated with the depreciation of the hardware. Even if mining somehow required no electricity, then the network would still be secure because there is some Time-Value cost associated with owning computer hardware.

>If the electricity cost to mine a block is low enough compared to the value that could be gained by a double-spend attack (which is of course comparable to total BTC market cap), then the network is not secure.

I agree. I think bitcoin will eventually succumb to selfish mining attacks.