how much security do we need ? It's hard to quantify security:
I think we should be ultra conservative and try to reach maximal possible security level.
Don't forget that those who have potential to kill bitcoins are not Trolls...
Maximal possible security in regards to node decentralization = every user have full unpruned node active.
This was never the plan for bitcoin as you can read from the original whitepaper and is not possible unless you want bitcoin to only be used by a small group of people which incidentally makes it insecure because you are keeping bitcoin
a small fringe project that could easily be squashed, thus because it is impractical to scale it actually leaves you with less security.
Personally, I don't agree with Satoshi's vision of most people using SPV nodes either and want the bitcoin network to remain more decentralized.
I don't think the problem has to do so much with a total node count but other factors like ASN distribution and different interests in control of the nodes. Thus a ratio of 1 full unpruned node per 10k users is fine if you have a ratio of 1 full pruned node(coming in 0.11) to 50-100 SPV clients distribution if and only if those full unpruned nodes are distributed in the right manner.
Thus when you look at this map of the 6.5k nodes -
https://getaddr.bitnodes.io you don't see a wide distribution per country. I would want at least 1 full node per country, and at least 1 full node in most of the large ISP's. Additionally, most of these full nodes are coming from miners and mining pools which further unbalances the power dynamic as they have already most of the influence because they control the hashing power and everything that entails so I would want the full unpruned node distribution to be controlled by a combination of miners, hobbyists , non profits , merchant processors, and businesses more equally. The reason this isn't the case right now is there is no incentive to do so, and the solutions to this problem where discussed in my previous post where I gave 6 different examples.
This only is addressing security of the network in regards to node distribution, and there are many other ways bitcoin is very fragile that need to be discussed but that is for another thread.
Do people agree with my opinion, why or why not with specific reasons cited?