Moreover, TeX has no serious competition (at least in the niche of typesetting mathematical documents). There are plenty of good text editors and IDEs to go around.
Lout (http://savannah.nongnu.org/projects/lout) and troff/groff (https://www.gnu.org/software/groff/). Both do equations quite well, I would even prefer lout (it is a way more comfortable than LaTeX), but the lack of additional packages makes it inferior. Troff is older than TeX (not sure how old is eq package).
Sorry, but eqn is just not in the same league as LaTeX. It's okay for simple equations, but as can be seen from the document you linked, it produces awful output as soon as the equations become a little bit complex.
