Mining bitcoin is essentially printing money, so miners with profitable setups have an incentive to expand these setups. As mining capacity grows, a parameter called "difficulty" - which represents how much power you need to waste to mine a block - is adjusted so that the mining rate remains constant. Since mining is necessary to maintain network security, difficulty will grow to compensate for expanding mining capacity, but not high enough to kill it off.
If you look at the history of the adjustments to difficulty, you'll find a pretty much clean exponential.
(In other words, Bitcoin is as close as we've ever come to expressing greed in units of kilowatt hours.)
Since supply will become more and more constrained over time, while demand has no realizable upper limit (that we know of), this assumption is wrong:
> Obviously bitcoin price won't grow to the moon and its price is limited by some economical factors.
If bitcoin were to be fully adopted by the mainstream, BTCs would be easily in the M$ range.
So that's a rough estimate upper bound if Bitcoin would become our civilization currency of choice: 1.2% of total energy spent.
Not the end of the world, if you ask me.
Here's my reasoning. Miners spend electricity. They're getting block rewards and fee rewards, they'll exchange earned bitcoins to dollars and pay their bills. So ultimately energy spent per block = (block reward + fee reward) x bitcoin price.
Right now block reward is 6.25 BTC. Last block fee reward was 2.13 BTC. Block reward will go down as time goes on.
I don't really understand economics of fee rewards and whether they'll grow over time linearly or exponentially. I checked historical data and it does not seem to grow at all. So I'll suppose that reward fee will not grow.
So in the end you'll have block reward of 10-20 BTC. That's all you have to pay your electricity bills after you mined block. So basically energy spent per block is directly correlated to bitcoin price, and that's about it.
Obviously bitcoin price won't grow to the moon and its price is limited by some economical factors.
So while bitcoin network might draw non-negligible amount of electricity, it still is limited by design.
Now if miners will decide to change bitcoin design, for example to increase block size, that's another matter. But it's not clear if that would happen.