"the amount of non-mining full nodes is absolutely no measure for the decentralization of a coin." (at least, with a PoW coin).
I already explained several times why I'm convinced of that statement, and that is because the only *source* of data, of block chain, is a consortium of miners, which these nodes can simply copy, and put at disposal, like a proxy server.
You POV is interesting - maybe the quantity of "non-mining user nodes" is really less relevant as most people think. However, I have two comments:
- non-mining user nodes are also useful because they add redundancy to the decentralized storage of the blockchain - if the 20 miners should go offline some day (that's not completely impossible, if we take into account that most miners are located in a single country - a large electricity blackout in China could be the reason for such a "catastrophic event"), then other nodes could jump in and serve as "data source".
Yes, that is true: as a kind of "collective indelible memory" non-mining nodes do have the function of "keeping the memory". However, one can wonder what that memory is worth, without miners. After all, we don't care much about a static block chain ; this is not something like, say, the "memory of the Holocaust" which has to be transmitted from generation to generation, and kept intact even though evil forces would like to modify it. No, a block chain only has a meaning as a dynamic entity that can grow. As such, suppose that all miners collude to change the block chain, and produce a new one, which has a modified history, and nevertheless, more PoW than the old "true" one. One can say that there has been a successful 51% attack on the immutability of the block chain. Ok, but suppose now that all miners agree with that, and that this modified chain is the only one available and growing. What is now the use of your stored, true, old, block chain, with less PoW than the current dynamical one, and which is not growing at all, which is a dead piece of data ?
Ok, at least you *know*. True. You know that the immutability of bitcoin is gone. So what ? If you possess, say, 1000 BTC (as well on the old, dead chain, as, happily, on the new one) ; are you going to consider them worthless, or are you going to try to sell them (on the new chain of course) anyhow ? What are most bitcoin holders going to do ? "leave bitcoin", or try to do as if nothing happened ?
Your scenario of a big power failure in China is indeed one of the useful cases of "a block chain copy preserved in New Zealand". Although of course, the miners in China may have a backup of the chain too... unless they have been bombed or something.
But let us imagine another scenario: a big *internet* failure in China's outside link. Suppose that the rest of the world is cut off from China's network for, say, a week. Suppose that nodes have copies of the block chain, outside, but almost nobody is mining. The high difficulty outside China and the low hash power make that only a few blocks are added outside. But inside China, people are happily mining, trading, transacting. The block chain grows almost normally in China.
And then, suddenly, the link is established again. What happens ? All the external blocks are orphaned. All external transactions didn't take place, after all. Whether this chain is saved on hundreds of nodes or not. The few Chinese miners with all their hash power have been building a longer chain with more PoW, and the outside nodes can throw away all their blocks since they got separated from them.
(BTW, if ever this happens, it is the moment to spend all your BTC if you are outside of course !)
So, the true power of "knowing the block chain", apart from a big bombing of all the miners and their backups, seems to be non-existent after all, apart for informing you.
- the discussion about UASF is showing that user nodes actually do have some powers: they can delay the propagation of blocks that do not fullfill some requirements (e.g. are not voting for their preferred soft fork solution). If only miners validate and transmit blocks in a kind of "miner intranet", then this (in my opinion) important element of the power balance (it reduces miners' power a bit) becomes unavailable.
My argument has always been that there is OF COURSE an "intranet of important miners" because every second counts. A miner wants to receive *as quickly as he can* the blocks from other miners, because otherwise, he's wasting hash rate mining on an old block. So every miner has all the reasons to be connected with a very good network connection to the most important other miners, or this will cost him important hash rate. No way a miner of any importance is going to wait multiple delays in the P2P network for the new block on which to start mining, while his competitors started already. All the seconds lost between the availability of the new block to other miners, and to him, is wasted hash rate.
Given the fact that "having a good link to another miner" is a mutually beneficial element for miners, there is in my mind no doubt that there is a direct backbone network of miners (at least, the important ones), and that thinking that the P2P network can "filter" miner blocks is delusional.