How does this change the threat model for using keybase? It would seem to increase the target value of compromising the client. Is that immediate impression wrong?
Yes. I don't believe Keybase will not store Stellar private keys in any way, this would be horrible.
what does prevent you from storing your private keys today? Nothing. The association with Stellar might imply a higher likelyhood of something valuable crypto related being there. However, I don't care, if the security of keybase is somehow broken, I'd rather find out early on and with the increased exposure, they may be getting free pen testing.