Would we really be able to apply "meters" to decentralization?

For a given payment system: there are middlemen present, or they are not.
It's a binary phenomenon...

In other words, it can only be that:
1) you have the freedom to send funds to any person you choose, receive funds from anyone you choose to send them to you, or hold funds for as long as you see fit
or,
2) you do not have this freedom.

So, for me, the factors that are important for maintaining decentralization (we assume that Bitcoin is decentralized) is the same class of factors as those describing the chance of death while performing certain activities.

Since life/death is also a binary phenomenon, and at the stake here is the loss of something indisputably valuable.

We could try to use the language/the tools of statistics here, most probably.