TeX is essentially hermetically sealed, device-independent software, all the way down to how the curves of the fonts are constructed. It's in some sense better than open source: all of the code is written in a precise, expository manner, than can be understood by any computer scientist 100 years from now. It can be purchased as a literal and markedly beautiful book, typeset in itself. It has virtually zero dependencies. It has been around now for ca. 50 years. It has a vibrant and passionate community of users. It's in the workflows of academic publishers.
TeX is a beacon of truly long lasting, unkillable, maintainable software.