[rthomas6]

I don't see what prevents updating as long as you don't care about fragmentation. You probably can't compile all brand new software on a very old Linux kernel, but who cares. I mean yeah, you'll have to care more about fragmentation, but it's not all or nothing. You'll still be able to update the protocol, you just have to make breaking changes less often.

I think XMPP is a better comparison than SMTP. In its heyday, XMPP had several clients, some with different proprietary extensions, and all the core functionality basically worked across all the clients. Though it turns out some of the messengers I thought were XMPP were actually different protocols that XMPP could work with. Imagine that. People still use it too, though it's not as popular as it was in the 2000s.