hey @dougla thanks for your thought provoking considerations
I'm not sure your note-in-the-cement-bucket really represents what's going on.
Trust is not only guaranteed by the immutability of the blockchain,
Trust is also guaranteed by it's transparency: everyone that has the blockchain data can read that note, everyone,at any moment in time after it's creation (unless it's intentionally encrypted data or we'd use "starks", which for today are quite impractical)
The data in itself is not immutable, rather the history is immutable
in the sense that you can't revisit history and write in the history books (your note) that Germany won WWII, because there is a previous version of history that says otherwise and everyone who holds a copy of your historical note/book in their blockchain will contest your attempt and your version will not become part of the shared consensuated blockchain.
The data is mutable, you can retrieve the note today and add or completely change its contents, but you will never make disappear that a certain content was there at a moment in time (similar to wikipedia history) but decentralized.
As per decentralization, yes you are right that there are different levels of it, and it can be quantified, and has been measured both in Bitcoin and Etherum with chilling results,
but decentralization is definitively one of the pillars caracteristics of the blockchain (read it in the words of Vitalik Buterin "the Meaning of Decentralization")
As to answer the question in your title, I of course think that yes, users should care
I recently asked a similar question in the #1 Open Source Web3 Design Call to various people
and it emerged that: "it depends!"
[ Partial-Dapps ] don't show
If the Blockchain is only a support element to a larger application you don't have to sell them all that decentralization (like in the case of games, where maybe the blockchain is used only in the exchange of NFT collectibles, but it's embedded in a larger application where the blockchain is not seen
[ Full-Dapps ] do show
if instead the Blockchain is a key element of the value proposition of the Dapp, then yes it makes sense, and I would say it's fundamental that the dapps shows and communicates the values of decentralization, trustlessness and transparency, and that, if possible, they are brought up to the front level (like in the case of ENS, Status, Aragon, Golem, Filecoin, Gnosis, Augur, etc)
I also go as far as saying that, in this last case, the users will want to know that a Dapp is really decentralized because it's the value proposition that differentiates it from a centralized competitor.
I think that one trait among all users of Full-Dapps will be exactly that belief in decentralization (and the other values of the blockchain)