Having read the insightful Three Ring Circus Bitcoin explainer by Danno Ferrin (https://medium.com/@shemnon/bitcoin-is-a-three-ring-circus-1cbe8756a842), I’d like to suggest that in order to visualize the phenomenon that is the Bitcoin Blockchain, we might want imagine a type of 2D Chain Matrix, or even better, a Tree, rather than a single dimensional simple Chain.
As Danno points out, the Chain analogy works somewhat at first glance because a chain is only as strong as its weakest link.
However the simple Chain analogy breaks down when you consider that if you break an early link of the chain, that means you can separate the very recent portions from the earlier parts, just as easily as you can separate the earliest parts of the chain from each other, and moreover a longer chain gets more fragile rather than more robust!
Instead, a Blockchain resembles a living Tree organism reinforcing early parts of itself (the trunk) with additional strength while self-healing any injuries to its trunk as it grows from the top. The freshest growth of any tree can be snipped off with a simple pair of scissors, but these same scissors stand no chance of cutting down an old tree at the trunk!
The freshest and weakest nodes of a tree may look a little fragile and frumpy for a short while, before they get a chance to fully set into twigs then branches, just as with Bitcoin network confirmations.
In the tree analogy, you could use scissors to carve your initials in the tree trunk, just as given enough time, absolutely anyone can use Brute Force to break any number of old links in the Bitcoin Blockchain to try and reroute (double spend) old coin transactions, especially since the hashing difficulty was low in early times and our computers keep getting faster each passing day.
Despite this, just as a thick old tree is not in any real danger from a pair of scissors or even an armored truck crashing into it, the security of the Blocktree remains safe thanks to the fact that early portions all the way at the bottom get more solid thanks to the continuing growth at the top. The very creation of more recent growth reinforces the strength of the earlier branches and trunk.
I think it’s necessary to see this in order to fully appreciate the potential limits to Blockchain security: the tree can be seen as safe so long as the available attack vectors (e.g. scissors) under the control of any malicious actor are within the same realm of physical magnitude as the total power of new growth at the top of the tree (mining hash rate). This is why private non-Bitcoin Blockchains are just fancy Rube Goldberg devices — anyone with scissors could cut down a sapling, so any application requiring security (essentially all of them) requires the strongest tree on the block, in a self reinforcing cycle of adoption.
In keeping with this point, should a chainsaw level attack vector suddenly be focused on the tree for some time by a concerted attacker prior to chainsaws being available at the top of the tree of Bitcoin (mining hash rate), it’s conceivable that the tree may be vulnerable to being cut down at the trunk during such a gap, depending on the specific power ratios involved. With sufficient head start for the chainsaw wielding party, such as before some new revolutionary purpose-built tree cutting tools make it into general availability to the miners (as possible with the advent of quantum computers), reinforcement and healing of the trunk may not keep up with the cutting action of the saw. For the tree to survive, the recent scale of total new growth must be within the same rough order of scale as any single attack vector to push back in real time against any removal of material by the saw, essentially healing its thick trunk and rendering it capable of withstanding the attack before the tree is cut down.