If you ask me if bitcoin is decentralized, my answer is: "can they change the protocol ? If yes, it is centralized, if not, even if many would like to, then it is decentralized".
so it's literally cast in stone for eternity? that doesn't sound very practical. and whoever set the rules that cast it in stone in the first place could be thought of as a centralised decision too.
That's what I said: a decentralized system always starts out as a centralized one. Bitcoin was initially centralized with all power to Satoshi. He set the rules. And Core could live still a bit on that central authority, making the sole software.