To raise the cost of a compromise through distributed signing authority, we have to solve the trust management problem - getting developer keys out to end users in a way that doesn't involve trusting the central PyPI service. That's actually a really difficult problem to solve, which is why we have situations like TLS still relying on the CA system, despite the known problems with the latter.
I haven’t read the entirety of your email, but I would like to point out that PEP 480 does not attempt to solve this problem without trusting PyPI. Rather it just moves the trust from trusting the server that runs PyPI to trusting the people running PyPI itself. TUF is fundamentally extremely similar to the CA system except there is only one CA which is scoped to a particular repository (e.g. PyPI) and it includes some distribution specific stuff like file size and delegating partial trust.
PGP: 7C6B 7C5D 5E2B 6356 A926 F04F 6E3C BCE9 3372 DCFA