The R5RS Scheme specification w/ example code: 50 pages
XML 1.0 specification for structure only: 49 pages
Sun's efficient, binary XDR: 29 pages
Wirth's Oberon Programming Language spec: 21 pages
ECMA JSON specification: 14 pages
Jsmn - portable, JSON parser [2] in C: 7 pages
I'm sure the above illustrates quite well how difficult and complex XML is. Additionally, while students everywhere implement LISP, there's a whole chapter in Beautiful Code where writing a good XML parser is an accomplishment people pay to see. Far from minimal understanding, reading XML's specification is a HOW-TO guide for how non-programmers that haven't seen superior works (eg Scheme, Oberon) try to handle data management. It was horrible, I used s-expressions/XDR instead, and implemented whole thing in a few hundred lines of straight-forward code. Best of all, unlike XML, my code could've run through a tool to prove it free of the types of bugs hackers love to exploit.
So, there's my proof that XML is garbage. Essentially everything it does can be done better by using one or more tools with less complexity, more cost-effectiveness, and more straight-forward implementation. Nobody should waste time learning it. If work demands, they should just learn a library that handles the awful details for them and internally use something else. Also, they should avoid any protocol that depends on it: that's just bad design.