Comment by tptacek

SSL 2.0 was overhauled (by cryptographic experts) to create TLS, and, after something like 10 years of effort, support for SSL 2.0 was scourged off the Internet. And still, we had the DROWN attack a couple years ago, which manages to exploit cross-protocol attacks between SSL 2.0 and TLS on different servers. In the last few years, we got TLS 1.3, which again made significant, breaking changes with the previous versions of the protocol (such as getting rid of the RSA handshake), and presumably over the next 10 years we'll be shedding support for all previous versions of TLS.

That's what didn't happen with PGP.