[infinity0]

It's not "nominal"; losing private keys is a major security concern. No, "TLS is useless" does not follow from that - we have things like forward secrecy which are security models specifically designed to give some protection in the case of private key compromise.

(edit: e.g. see https://en.greatfire.org/blog/2015/sep/popular-chinese-ios-a... just out today)

Especially in the case of a library developer, they hold the keys to many websites, so there is extra incentive for an attacker to break that rather than "some random guy's website". The more third-party signers you trust, the more holes you (and your users) have.

Furthermore, you are forcing your users (who actually run this code) to place their trust in these parties too, which is not a great thing (transitive trust) to force upon someone. (This is not the case for e.g. depending on system libraries explicitly installed by the user.)