TeX is written in a literate programming style which is more akin to a math textbook than ordinary computer code, except with code blocks instead of equations. The actual programming language in the code blocks and the OS it runs on matters a lot less than in usual code where at best you get a few sparse comments. Avoiding bit rot in such a program is a very manageable task. In fact, iirc the code blocks which end up getting compiled and executed for TeX have been ported from Pascal to C at some point without introducing any new bugs.
The C version of TeX is also terrible code in the modern day (arbitrary limits, horrible error handling, horrible macro language, no real Unicode support, etc. etc), hence LuaTeX (et al.) and Typst and such.
The backward-compat story is also oversold because, yes, baseline TeX is backward compatible, but I bet <0.1% of "TeX" document don't use some form of LaTeX and use any number of packages... which sometimes break at which point the stability of base TeX doesn't matter for actual users. It certainly helps for LaTeX package maintainers, but that doesn't matter to users.
Don't get me wrong, TeX was absolutely revolutionary and has been used for an insane amount of scientific publishing, but... it's not great code (for modern requirements) by any stretch.