I have a hard time imagining a proof of work based distributed system that isn't guaranteed to be monopolized after a sufficient amount of time.

Work -- any form of work -- is subject to industrial scaling effects and economies of scale:

https://en.wikipedia.org/wiki/Economies_of_scale

Meanwhile all these POW-based currencies have deflationary economics and deflating mining revenue due to rising difficulty. That makes less efficient mining less profitable over time, driving smaller miners out of business and accelerating the effect. Any proof of work based system like Bitcoin or Ethereum will in the end be monopolized by whomever has the cheapest electricity, cooling, and hardware, and the most labor-efficient mining operation.

This is the most powerful bearish argument on Bitcoin and other cryptocurrencies that I have. Gold and other hard currency materials are randomly distributed by geology and physics, helping prevent total monopolization of mining. Paper currencies are monopolized by default, so you know it's controlled by the Fed or some government... the devil you know. But these things are guaranteed to be monopolized by... uhh... we don't know yet. You could end up with a currency controlled by Kim Jong Un or Dr. Evil and his white Persian cat. (I'll take Dr. Evil.) Serious global organizations are not going to adopt a reserve currency whose future control is up for grabs by whomever can build the most efficient data center and/or snow over the development community most effectively.

That's why Chinese miners have so much power on the Bitcoin block chain. Cheap coal and a total lack of an EPA makes power very cheap for them, and they also have local access to very cheap labor and cheap hardware production.

In the very long term if Bitcoin or any other POW-based currency became cumulatively worth enough, someone in a chilly place like Russia, Scandinavia, or Canada could build a hydroelectric powered arctic circle (free cooling) super data center full of ASICs and own the entire block chain. There are places on Earth with unbelievably cheap power (big hydro, constant prevailing winds, etc.) and free cooling due to a cold local climate. A coastal location in the arctic using arctic seawater for cooling and a dam on a local river would be ideal. Nobody would be able to compete with that.

Bitcoin has two big things going for it, at least insofar as the mainstream market is concerned:

1) The 2008 financial crash and its terribly bungled and corrupt aftermath destroyed peoples' long term faith in the "devils we know," creating essentially a market niche for a competing financial system that offers better governance and transparency. Bitcoin, Ethereum, etc. and the many companies working in these ecosystems are trying to address that niche.

(Side note: I wish the West's leadership would wake up and realize just how profound a loss of confidence occurred in 2008-2009. I don't think the effect has manifested yet. It's too deep and profound a loss of faith and the population is still processing it.)

2) Existing methods of sending cash overseas like Western Union are absolutely freaking horrible. We've used Bitcoin to pay overseas contractors because it amounts to only one root canal vs. Western Union's three or four tooth extractions with pliers per payment. Bank wires requires that I physical visit a f'ing physical bank, which is even worse.

That means though that Bitcoin's advantage would disappear if:

1) Entities like the US Federal Government and the Federal Reserve took actual steps to reform themselves and implement a plan of action for dealing with things like the 2008 crash that is more honest, equitable, and well thought out than "yank money out of our arse and hand it to other Yale alumni until the fires go out."

2) Companies like Western Union realize they are in the 21st century and act accordingly, implementing low friction easy to use methods of wiring money with low fees.

Bitcoin would still have a niche if those two things happened, but its niche would be smaller and probably further outside the mainstream of commerce.

Of course if Bitcoin ends up controlled by the Chinese PLA, none of those above advantages might matter much.