Why is it a fetish?, if something is centralized it means you need to trust that central party. Decentralization removes that dependency

In the bitcoin's case, to verify the immutability of *any block*, you only need some basic knowledge of cryptography, and all the blockchain after that block. You then need to sum all the proofs of work of every subsequent block and divide by the network's current hash power, that will give you the amount of effort (energy, cost, etc) needed today to create that amount of proof of work, and that's how you get a feeling of the mind-boggling immutability of bitcoin's block

In summary, you can, independently, verify how much work a piece of information in a bitcoin block has. You don't need to trust anyone

Decentralisation simply moves trust around the system, rather than eliminating it. It is a fetish because most people really don't care.

Immutability is by consensus, not some fundamental law of the universe. Consensus can change (see the reason Eth Classic exists).

Bitcoin's guarantees are not mind-boggling, and the amount of work it would take to fake is not mind-boggling either, it's very quantifiable, and it is some fraction of the available computing power that exists today, for a time period of no more than the age of bitcoin. Compare that to the stronger guarantees of things like AES encryption, if you want your mind boggled.

I don't give a crap how much work a piece of information has attached to it, all that is to me, is a record of wasted cycles.

Further, in most real-world situations having no central authority that can make changes coupled with immutable transactions is actively terrible.