that's such a good question
I think @blocktivity could make a metric of decentralization by taking into account an equation with variables including Activity Transactions, Stake based Ownership ratios, number of users, and marketcap maybe and find a number that can show us data about HOW many users at the TOP own HOW much, how balanced the distribution is and I'm sure there's some great math for that, like coinfair or https://blocktivity.info can create!
I JUST read a comment a few minutes ago on an old @dan larimer youtube video talking about how EOS before it came out, and the 20 witness/ block producer system reminded them of the Federal reserve lol and it is sort of like a decentralized federal reserve with the top 20 families, and they are all voted in by shareholders with shares distributed to ensure they all in power due to a form of paper and pen "ninja mining" back 100 years ago hah