For the first thing we should understand why decentralization matters to us so much.
You want an immutable and permissionless trustless system, ideally anonymous. If the system is not immutable, then the one that is able to change the rules or the history at will, once you got in, can totally alter the value you are holding, or the things you planned to do with it. He can even change your balance, or wipe your existence on the system. Without immutability (of rules and history), you are doing the equivalent of signing a blanc cheque, or a blanc contract, to whomever has the ability to change the rules or the history.
If the system is not permissionless, one can kick you out, or stop you from using the system according to the rules, for your political, economical, religious, racial or social cercle identity, or simply because you happened to annoy for a known or unknown reason, those that can grand permissions or not. So the system must be open to any participant.
==> essentially, those being able to give permissions, to modify history or to modify the rules are a power house ; in the end power always converts to monetary and hence value advantage. This is why you don't want that.
Because the system's role is to be able to do what the powers that be don't want you to do, or don't want you to do that easily, or because your using of the system may be frowned upon by the powers that be who have all the means to make your life miserable, using the system should be possible without giving out one's identity.
But this also puts the problem that because everybody can access the system, without identity check, that the system most resist Sybil attacks, and of course, malicious people wanting to bring the system down, or take over the power over the system.
The system cannot have any leader, capable of changing the rules of the history (power house) nor anyone deciding upon permission to use the system or not. As such, you are obliged to have the system running by every participant, as there cannot be a centrally run server, with a root owner, that could change the history on it, change the rules by which it functions, allow or disallow participants in the system, and be able to know all (network) identities of all participants and their actions.
==> necessity of a decentralized system, in order to obtain permissionlessness and immutability, and the lack of any form of centralized leadership. But this leads to the necessity of trustlessness and resistance to corruption or Sybil attacks.
We are actually looking for the security of the network, not necessarily decentralized solution for the security of the network
Quick example: if we send centralized payment processor server to Antarctica and bury it under miles of ice no third party interference can happen to payment system, so centralized network would run secure enough.
How do you know that server is really under the ice, and not in the room of a power-hungry maniac ? And what happens if that computer fails ? Who has the root password to that server ? Who can pull the plug ? What network provider has control over all that happens on the network interface of that server (excluding people for instance) ?
We have bitcoin block solving process decentralized to 20 computers, with hardcoded checkpoints, and network run fine.
Who can control those 20 computers ? Who is deciding on the "hardcoded" (who is coding them ?) check points ? What if tomorrow, these 20 computers are running an entirely different block chain ? Is there even a block chain on them, or is it just a database pretending to be a block chain ?