Here's how to justify it: If you want to talk about something (program), you want the right language. We've got powerful languages capable of everything any computer can do, but new ideas often call for new ways to express them.

Building a language is about accepting massive up-front investment to build something that hopefully meaningfully improves user experience. Or you can build to learn about the inherent beauty of computation and formal languages, but I find that a little less compelling.

Unconvincing article. PLT is a theory subject, as it says right in its name. If you're a practitioner and don't get off on theory, you're not going to be designing bleeding edge languages or compilers anyway, so you can stay pretty hip by just using the theory-adjacent stuff that comes out of the research places (example: Haskell). Just like if you're an electrical engineer, you might want to keep up with new kinds of semiconductors, but you don't need to study bleeding edge physics.

If you want to get started on PLT, Harper's PFPL is pretty accessible (https://www.cs.cmu.edu/~rwh/pfpl/). Even Martin-Löf's article on intuitionistic type theory (the one that introduced dependent types) is fairly readable for a PL geek.

I'm unfamiliar with Barendregt's article but it sounds too mathematical by comparison. I.e. by the title I'd classify it as mathematical logic rather than PL. Remeember there were no computers when Alonzo Church invented lambda calculus in the 1920's.

Programming language theory has been successfully taken over by functional programming, while programming languages in practice, those used by people, are largely not functional languages.

This means we users, and specifically for me, compiler developers rarely get to use the fruits such theory and research, making tools a but janky in practice.

PLT is an interdisciplinary field and as such needs a study of various logical/mathematical/language areas. Even wikipedia is pretty bad at explaining what it is - https://en.wikipedia.org/wiki/Programming_language_theory.

The research community has failed the "ordinary" interested programmer wanting to learn PLT by not producing an overview/top-down book which brings together the various strands of needed knowledge into a coherent whole. Pierce/Harper/etc's books are simply overwhelming for a beginner without the requisite background knowledge.

PLT approaches the idea of Computation from a formal system (i.e. language) pov and that is what needs to be communicated with the needed logical/mathematical knowledge inline. I have found the following useful in my study of PLT;

Formal Syntax and Semantics of Programming Languages: A Laboratory Based Approach by Ken Slonneger and Barry Kurtz - https://homepage.divms.uiowa.edu/~slonnegr/ and https://homepage.divms.uiowa.edu/~slonnegr/plf/Book/

Understanding Formal Methods by Jean-Francois Monin.