Please just stop and think for a moment FFS.

Miners care about where their bitcoin comes from. Let's imagine the case where 100% of their income comes from the block reward and 0% from fees (seeing as this is nearly the case right now):

1. the block rewards are getting CUT IN HALF every once and a while. If the price of bitcoin stays the constant, their rewards in terms of dollars will be CUT IN HALF. The only way for their income (the upper limit on they can spend on electricity, hardware, etc.) to increase is for the price to increase by more than DOUBLE every time the mining reward gets cut in half.

2. the block rewards increase the supply of bitcoin, decreasing the market cap/price over time. It depends on the emission schedule. Even if the block reward was a constant 1 bitcoin per block, then you would just have a different (logarithmic) inflation/block reward graph with the same characteristics.

Let's say 100% of income comes from block rewards of 1 BTC. If it costs 0.5 BTC to mine a block then miners will beef up their mining capacity so they can mine more. In a vacuum, this would be great, but because of how mining works (winner takes all), the cost of mining would go up as the number of miners increases. This will keep happening until the cost to mine is close to or equal to 1 BTC (and because of currency conversion, the cost might even exceed 1 BTC today but miners are hoping that the price goes up so they can sell at a profit).

Let's say tomorrow, the income is cut in half. All of a sudden, most of these miners would become unprofitable and stop. However as a result, the cost of mining will go back down to 0.5 BTC. It's a self balancing system but that also means that it's fundamentally flawed for reasons mentions by previous commenters in this thread.

I think you are talking past each other. Leaving out transaction fees for a second (which are currently a very small percentage of the miner reward). The block reward is programmed to half every 4 years, taken alone this together with a fixed market cap would mean that the mining electricity spend would also half every 4 years. So the energy consumption of bitcoin will grow only so long as its market cap also doubles every 4 years. That's a tall order.

Bringing back in transaction fees-- I don't think anyone really knows what they will be, but it will be a long time until the dynamics of that are more important than the dynamics of the block reward halving and the market cap.

> At the end of the day, it's a very simple calculation for them: electricity costs in, Bitcoin value out. No matter where their Bitcoin reward comes from, they will not (for long anyway) spend more on BTC then they are rewarded.

True, but the hypothesis in your original comment, that energy expenditure will equal Bitcoin total market cap, does not follow. As other commenters here note, with each block reward halving, the energy required to mine a new block becomes more and more decoupled from the price of Bitcoin itself. In the distant future when block rewards are minimal and fees make up the majority of miners' rewards, only miners who can utilise the cheapest sources of energy will be able to mine profitably, which in the "negligible-block-reward" era will be for the lowest fees. And people making transactions will not pay more fees than they need to, so the existence of miners able to mine profitably for lower fees will in turn bring down average fees. I think all of this should be uncontroversial; it simply follows from the Bitcoin protocol and the work of Adam Smith.

So, given these incentives and Bitcoin's difficulty adjustment mechanism, in the future the only miners capable of making profit from mining will be those with access to the cheapest forms of energy. Anyone who can mine for marginally lower cost is going to push out other miners. This will likely mean the majority of the energy used for Bitcoin mining will come from otherwise wasted energy - such as flare gas, remote hydro not economical to transmit to where people actually live, excess wind and solar supply when demand don't line up, that kind of thing. (And, as an aside, it's probably not going to involve burning lots of oil and gas, since these are more useful to humans in other ways and will therefore cost more.) Obviously more efficient hardware will have the same effect, so there will also be an incentive there. In the end it will have very little to do with Bitcoin market cap and far more to do with the availability across time and space of cheap energy.

> And people making transactions will not pay more fees than they need to, so the existence of miners able to mine profitably for lower fees will in turn bring down average fees.

Only to a point. Since a limited number of transactions can fit into a block, users competing to get their transactions mined in a timely manner will drive the fees up. That price point can be well above the cost of many forms of energy.

So as long as Bitcoin is still valued and there are enough people wanting to make transactions, there will still be miners burning useful energy.

There is still some coupling over time - the miners' bitcoins depend on transactions happening to retain their value