In fact, as long as these 20 entities are not attackers, and they have never been
As of today. But that "20" will continue to shrink until someone can control 51%. It's inevitable assuming "all things" stay the same.
Well how about this alternative: one corporation controls the trajectory of bitcoin protocol and network, and we're supposed to magically trust them with this responsibility, despite their sketchy and counter-productive behavior? *cough* Blockstream *cough*
Even if only one person "controlled" bitcoin, who is to say that they would want to run it into the ground?
Indeed, the true centralization of crypto in general is linked to who can decide about the protocol. MOST crypto is heavily centralized in this respect, because most crypto has a lead developer that decides about future evolution of the protocol. We saw this clearly with ethereum where the unthinkable in crypto happened: modifying the past, not by modifying the block chain data, but by modifying the protocol that tells you how to interpret that block chain data. Many other coins have such single dev team: DASH with Evans, LTC with Lee, of course ETH with Vitalik, ...
If you can change the code, you can change all of it, no matter all the "crypto".
In fact, bitcoin is decentralizing by not letting Core just do what it wants on this side ; but it is centralizing in the mining industry.
The point, of course, is that if there's a limited power structure in crypto (that is, if the deciders can sit in one room and come to an agreement, like they did on litecoin), it is a kind of "governors of the central bank". Governors of the central bank also don't want to run the fiat they command, into the ground.