It's not correct that you need to trust Keybase. The way that someone verifies their social identity is by posting a tweet (or equivalent) signed with their private key. So you can look up someone's public key on Keybase and then verify that Keybase gave you the correct key, by checking the signature on their original tweet / other social verification posts.

Assuming you actually do this level of verification yourself (rather than allowing keybase to do it for you), the only thing you have to trust is that Keybase/Twitter/Github/Facebook/etc aren't all simultaneously colluding to give you a bad key. That seems to me like a reasonable assumption in pretty much all plausible circumstances.