Let's talk about how Disney Studios lost their way. They tried to continue the methods of the great man after he died and it was a disaster. Then they suddenly realized that the great man himself had never done things the way they thought he did. Walt Disney did use storyboards, they were in his head all the time. So now they use storyboards and if you invested in Disney back in 2000 you are a very happy camper right now.
We get the end-to-end principle wrapped around the axle in the same way. The argument is much more subtle than most imagine. The real principle being *think* very carefully about where you put complexity.
The problems with the robustness principle became clear when we started to try to extend HTML and found that it was almost impossible to do it right because every implementation in the wild handled unknown input in different ways.
The issues with SMTP are not the same because we have never really tried to change SMTP in drastic ways and even the incremental extensions are all working around the mass of legacy deployment.
My approach is to distinguish reference code from running code and reverse the robustness principle. Reference code should be pedantic in what it accepts and liberal in what it generates. In fact it should perform fuzzing on its outputs deliberately sending maliciously formed messages to test for security vulnerabilities.
We live in a different Internet today. There are well funded nation state actors working to break it. That is their job. There is a full blown cyber-war going on out there.
The other point I think relevant is that there are limits to re-use of existing code or infrastructure. There comes a time when starting from scratch is not just the best approach, it is the only viable approach. Accepting the robustness principle means this point will be reached sooner.
