Fact: if the entire consensus mechanism as it was designed, is decided by 20 entities, it is not a peer-to-peer CONSENSUS SYSTEM any more. And that it the current reality.
Which, sadly, implies it's a failure. "The system is secure as long as honest nodes collectively control more CPU power than any cooperating group of attacker nodes.". Course one could "argue" about what the definition of "attacker" is but it is what it is at this point.
I think it is quite clear what an attacker is in bitcoin's case: it is an entity wanting to modify the published consensus history.
In other words, a miner building on top of the chain is never an attacker (a miner orphaning the last block isn't, either). Someone trying to overdo the last 50 blocks, however, is an attacker.
I think one could define an attacker as someone orphaning ON PURPOSE any block older than 6 confirmations.
It is here that proof of work is a terribly BAD cryptographic security. You can't find worse. In as much as consensus FINDING can be done with just any sufficiently fair and random way, past consensus securing should be done with better cryptographic means than proof of work.
In fact, as long as these 20 entities are not attackers, and they have never been, the system is not a failure in the sense that it allows people to transact. But it is not a peer-to-peer system, and it does have points of failure (for instance, law enforcement intervention). But it seems to work. Not as it was conceived, but it works.
As such, my idea is that given THIS deviation from the original P2P vision, the discussions concerning "full nodes in your basement" are after the battle. The system is not a P2P system since quite a while. So if these entities are the ones deciding on the whole bitcoin consensus, they may just as well also operate the full nodes that everyone connects to with light wallets: it won't change the power structure of bitcoin's industry.