If Lisp flourishes, more people would get to use it at work.
Lisp not flourishing means less fun.
Also, when I've worked in Lisp it just seemed like the right way to do things. Everyone likes to do the right thing, so anguish results when you seem denied the opportunity to follow the path you feel is right.
That said, I think the author was right and the community issues and ease of building your own solutions will limit Lisp as a language family.
Because it's fun.
Is it possible to do the same in another language? Sure, Slack is proof that it's possible. But would I have persisted in actually doing it without any hope of profit? No, because programming in other languages is a chore, and the only light at the end of that tunnel is the finished product.
With Lisp, the journey is a reward of its own.
Yes, I know the same thing has been said by Haskellers and others, but I doubt it was ever said by a C++ programmer.
APL is less extensible in this regard (to my understanding, someone might prove me wrong). Smalltalk is probably on par, if you can model it with objects and methods/messages cleanly.
But in the end, for me, it's the idea of programming the AST directly and being (mostly) freed from arbitrary syntactical constructs.
To me, this hits the nail on the head, for all the wrong reasons. Syntax isn't the problem, implementing the feature is!
If I'm writing Lisp code, why do I want it to look like Erlang? What's wrong with it looking like Lisp? ("Erlang style concurrency and message passing" is two things, syntax and functionality. Do you want the functionality? Fine. You want the syntax too? Why?)
In fact, this is kind of what the article is about. People want the syntax "their way"; it makes it hard for others to work with their code.
The only time you should invent new syntax is when it is impossible (or very hard) to say something in the syntax you've got. "I don't like it" isn't good enough. "It will save me a few keystrokes" isn't good enough.