It's undeniable however that, as of Emacs 25.2, Eshell has numerous rough edges.
And this sums up exactly why it isn't used more widely. According to the changelog, Eshell has been in development since 2000 and still has "numerous rough edges", up to and including "doesn't support INPUT REDIRECTION". Even the list of "bugs & issues" in the documentation refers to Emacs 22 (from 2008). Not that newer software is always better, but users can't look at that timeline as reassurance that the bugs will ever be fixed.
I'd also find arguments like "you don't need THING_X, you can just use Emacs!" way more convincing with examples. One person's "trivial computation in Lisp" != another person's.
I'd also find arguments like "you don't need THING_X, you can just use Emacs!" way more convincing with examples. One person's "trivial computation in Lisp" != another person's.