I'm an associate professor of computer science, and my PhD was in cryptology. I think I know a bit about what I'm talking about here. Please stop assuming people you are talking to are just "not getting it" and try to make an actual effort to think about the answers you get.

I'm not talking about other consensus mechanisms. We were first discussing what makes a blockchains immutable, I already explained why PoW is not necessary for that. Then you reframed the discussion to how can you trust that a copy of the blockchain you were just given by someone you don't trust is valid, and you are right that PoW is a solution to this specific problem. My point, again, is that it is an unnecessarily costly solution for this specific problem, because the properties that makes a blockchain immutable (Merkle structure + distribution) are already enough to ensure that in practice, because a fake blockchain copy, even without PoW, will have a different last hash from the real one, and it will be easy to see that by simply comparing it with others sources (if you trust no one, which is one of the requirements for blockchains to be useful, you just need to do that with multiple sources to gain enough confidence, typically ones that are very public and would be easily denounced if they lied). And anyway by participating in the blockchain protocol it will be obvious very soon that your copy doesn't match with everyone else's.

The only way this strategy won't work is if the last hash of the Merkle structure is valid while the blockchain copy is not, and the only way for this to happen is if a full collision has been found on the modified block. Which is way more costly than building a PoW valid hash.

Let me take an example with another technology that has the same immutability properties as blockchains do but don't use PoW: Git.

If you want to get a copy of the git repository of a project and you clone it from my version of the repository, either it is the real repository and everything is fine, or it is not —for example I could have introduced a backdoor somewhere in my version of the project— and then there is two possibilities:

1- Using the copy you downloaded from me you won't be able to participate (push or pull) in the project with anyone else than me because the commit log of the copy you got is incompatible with everyone else's version of the repository. You'll quickly understand that something's wrong with mine, and it will even be easy to see at which point the commit history diverges from the other copies of the repository that you attempt to collaborate with.

2- I've added my backdoor in a past existing commit, and found a collision in its hash to make it have the exact same hash as the original version of the commit (the one without my backdoor), and I have rebuild the exact same commit history from there. But here it requires me to have computed a full collision, which is actually impossible (at least much more so than computing a valid PoW hashes for a few commits after adding my backdoor), as long as there is no vulnerabilities discovered in the hashing algorithm.

Now, if you never interact with anyone else than me, I don't have to find a collision because my commits hashes won't be compared to any others, and then you are right to say that you won't be able to know about the backdoor and that using PoW would make this scenario less plausible (not impossible, but way less plausible) in terms of cost for me. But, even more in the case of bootstrapping a blockchain client, it is the very idea of only interacting with the person you got a copy of the blockchain from that makes no sense.