these
don't seem to have impaired their reliability,
performance, or sheer longevity: Being the 30-year de facto standard in
academic publishing.
That's because practically noone can understand it besides Knuth, which
is why noone has been changing it. Software that doesn't change
doesn't have bugs introduced into it.
Hang on a second. Even if you don't like litterate programming, the fact
remains that the source code of TeX (and Metafont) is avaialble, as is an
explanation of what is going on. If there were serious bugs in it then
sombody who have fixed them by now.
The fact that TeX doesn't keep on changing (and is still widely used)
seems to suggest it was 'done right' in the first place. Unlike some
pieces of software I could mention...
-tony