Re sendmail, when were you working with that? My reaction was just to look at it, say "nope", and used Exim instead. Perhaps the most instructive lesson here is the importance of good choices when it comes to selecting systems to depend on.
Other than that, I'm not sure what the lesson is in "people collectively decided to depend on one of the worst alternatives available." We still see that today with programming languages.
There's nothing really wrong with YAML, except perhaps the way some people use it. I classify that as "skill issue". I work with Kubernetes regularly, and its YAML usage is fine.
Something similar applies to JSON. If it's so terrible, what's better? With JSON Schema and OpenAPI, it's feature-comparable to XML now.
The problem with XML is its completely unnecessary verbosity outweighs its usefulness. I can only assume it was designed by ex-mainframe people who, unlike me, actually yearned for a return to the overengineered environments they were used to. It's no surprise that JSON and YAML edged out XML.
Emacs Lisp is an abomination. Sure, Lisp has its place historically - I had a spirited discussion with John McCarthy about that at a Lisp conference in the 2000s. I'll just mention two words: dynamic scoping. They took decades to even figure out a solution to the funarg problem, and that still didn't really fix the language. Luckily Guy Steele came along and noticed that Church had solved that problem before computers were even invented.
