Will the energy cost and deflationary nature of Bitcoins doom the currency to obscurity? I couldn't find enough questions that ask this head-on1, so consider that if:


Bitcoins are designed to reflect the deflationary characteristics of a specie currency; and 
The distributed nature of the cryptocurrency requires a high intrinsic value (energy cost) of the coins to secure them; and
That neither deflationary contraction nor high transactional energy costs are desirable qualities of a major currency (see Background below);


Then: 

Will the energy cost and deflationary nature of Bitcoins doom the currency to obscurity?

Off-blockchain solutions are acceptable answers, but will require centralising a large portion of the Bitcoin network. Or put another way: Trust moves back to centralised banking facilities.

So my guess is these two problems will doom a decentralised Bitcoin, but I open the question to the floor. :-)



Background (Due Diligence)

A high transactional energy cost is bad


  The green line in this chart reflects the power per dollar in the
  global economy; of which a tiny fraction is consumed by maintaining
  the banking network.
  
  
  
  (Source: Dr Tim Garrett, "A physics based economics
  model")
  
  As the distributed Bitcoin cryptocurrency doesn't use laws or other
  centralised off-blockchain enforcements to protect the transaction
  ledger, the energy cost per transaction can not be a fixed constant
  like normal currencies. It must be a percentage of global energy
  consumption2 to discourage state actors with a bevy of
  power stations from subverting the global Bitcoin economy.
  
  No society will choose a currency that requires O(N) energy to
  transfer a bank note instead of O(1) energy to transfer a bank note
  (where N is global energy consumption). Doing so would impoverish
  human development3.


A deflationary currency is bad


  Long story short, a deflationary currency requires a post-growth
  social model; which while probably a good thing, we don't actually
  have right now.
  
  In a classic "growth" economy, you must expand the money supply to
  reflect the size and velocity of economic activity in society. If you
  do not and the level of economic activity ever contracts (and there
  are variety of regular reasons this may occur such as a natural
  disaster or demographic bulge), then you can enter a deflationary
  death spiral as each contraction contaminates other economic activities
  that assumed an ROI in the form of interest that is now worth less or
  worthless4. 
  
  A digital specie-based currency is deflationary in that it restricts
  the money supply. 
  
  Inflation and hyperinflation are problems, but
  problems of unchecked greed, speculation and social inequality that
  don't disappear by sweeping the economy under a deflationary rug.




1. Although "Will deflation destroy Bitcoin?" and "How much energy will the Bitcoin network eventually consume?" come pretty close, I really want a question that collates and settles this for newcomers, without undue eager puppy optimism from early adopters. :-)
2. Production actually, but sane organisms consume energy they produce instead of throwing it away; so global consumption and production can be considered linearly proportional, though not 1:1 due to transmission inefficiencies.
3. To put this in perspective, you could provide everyone on Earth with universal education, healthcare, social security as an O(N) energy cost. 
4. Don't get me started on economists of the Austrian-school. They might as well be sock puppets for the entitled mega-rich for all the benefit their pseudo-science has on ordinary human beings.
On the deflationary death-spiral part:

The bitcoin wiki has a good discussion of deflationary spiral, so that's a good place to get a counter point. 

I'd like to make some of my own (amateur) observations:


Gold is currently not the most popular money used in the world not because gold is "bad" money, but because:

a) Laws make it more difficult/expensive to use for day-to-day transactions then nationals currencies (e.g. legal tender laws require capital gains tax to be collected at the time of purchases in gold), and; 

b) Banks are incentivised to use fiat through the fractional reserve banking system. Why hold gold for customers when you can hold national currency and generate an order of magnitude more interest by loaning out 10x the deposit amount that is guaranteed by the central bank?

To put it another way, the absence of gold used today for your average transactions is not because citizens fear deflationary death-spirals, but because governments have purposefully made it less beneficial to use gold.
Bitcoin has been around long enough that we can at least get a sense whether falling prices has been a disaster. As far as I can tell that is not the case. All the major bitcoin  merchant services like BitPay, CoinJar and Coinbase record ever increasing purchases/sales made through the website. Yes, more people are using bitcoin, but one person spending money in the bitcoin economy is counter to the death-spiral theory. If the theory is correct, no-one should be spending bitcoins. 
The deflationary death-spiral, when you think about it, describes something that in layman terms sounds pretty awesome. Theoretically prices will keep dropping to $0, meaning at some point, everyone on earth will be able to buy everything and any service for 0. Sure, the theory states that e.g. everyone will lose their job, but who cares when everything is free! At that point you don't need a job.
When I've heard the (in)famous cases of deflationary death-spirals mentioned, they seem to always conveniently ignore the broader context. For instance, the Great Depression happened at a time when the government purposefully reduced the money supply, i.e. it wasn't like the money supply was stable or predictable (like gold/bitcoin). In other words, I'm yet to hear of an example of "bad" deflation that "just happened" during a normal period of dropping prices. There's always something else going on in the economy.