Nobody uses (Knuth's) TeX anymore. They use PDFTeX, XeTeX and LuaTeX. None of these are as stable as TeX, but in practice a whole lot better than most other software I use.
All of these versions of TeX are just extensions of the original TeX. The core Knuth's code is still used in each one of them. The stability comes from the core, not just from the work of pdftex, xetex and luatex developers.
The stability comes from the core, but the problems arise from the extensions. I have pushed LuaTeX to some limits and seen lots of segfaults and other errors.
