In a way, bitcoin (and almost all other crypto coins) are still some what centralized. The devs are the centralization. There's not much we can do about that.
Yes, they are centralization. But as long as they do not adjust blockchain on regular basis (select which chain is good and which is not) - they gain trust of users.
The redeeming thing is that almost all of them are open source with code available to be self-compiled.
Yes, but the problem is that the client with checkpoints and one without - are not 100% compatible with each other. Checkpoints are not just cosmetic change.
While the protocol itself is decentralized, it is a requirement that honest nodes form a consensus. Remember the hard fork early this year.
Bitcoin already has mechanism to achieve consensus - voting by computing resources, it should not be substituted by any centralized authority.