Stablecoins are literally central banks. Decentralised coins are literally decentralised central banks. Bitcoin's algorithm is a central bank.

Well I don't think they're very decentralised, which was the original premise.

Any rules can and have been changed by those with leverage to do so (be that exchanges, miners or devs), in the end any system like this storing value is captured and controlled.

They are decentralized cause they operate on a sort of consensus between nodes as was original premise. Are they "very decentralized" depends on your definition of "very", of course, and techical realities of a particular situation.

Decentralized central banks - are these the finance equivalent of dehydrated water ?