"Centralised Crypto" in what sense? Unless I'm very much mistaken, your private key never leaves your control - signing/encryption is done client-side.
One _can_ host their private key(s) on Keybase (you need to to use in-browser tools) but you don't have to; can instead use Keybase cli, or gpg directly.
The entire system is architected so that you don't need to trust them for anything, beyond the trust you generally need to have in open-source developers. What is a specific scenario that concerns you?
Are you talking about the announced file storage service here? Or about the "drudru claims to be foobar on Twitter" service?
The former should be good - that sounds like a smart client in front of a dumb server. The latter is a different problem and as far as I'm aware the service is merely aggregating proves - and can link to those. Verify them?
