The original Bitcoin white paper explains this pretty well. The Bitcoin blockchain is never frozen, and the longest (the hardest) chain is always the valid one.

The original Bitcoin white paper explains this pretty well. The Bitcoin blockchain is never frozen, and the longest (the hardest) chain is always the valid one. The paper includes a formula to calculate the probability of a block being replaced by another, and sets 6 confirmations as the frontier where it is almost impossible for the chain to change. What the paper doesn't say is that the formula doesn't work if there is more than one chain. Since an ETH miner can easily switch to ETC at any time, proof-of-work isn't really protecting ETC at all. There is another chain that was a lot harder to calculate, which means the ETC chain stands on thin ice. Why not freeze the blockchain at certain points, then? Well, it cannot be easily done. There is the problem of choosing which blocks should be frozen, and when. How do you calculate that? What if there's a network split at that time? What happens if the protocol results in an unwanted fork? What happens if malicious actors try to settle on a wrong chain? This is the very problem that Bitcoin solved. Forgetting the idea of a final truth, and making the blocks work like a wave of consensus that eventually reaches everyone. Clever and simple, but with some drawbacks that have to be understood. Of course, there are lots of people working on new ways of establishing decentralized consensus. The Ethereum devs have been working hard on this for a few years.