I think we must have some software in use for which the compiler or the source code just isn’t around anymore. It probably isn’t a massive problem. There’s just a slow trickle of tech we can’t economically reproduce, but we replace it with better stuff. Or, if it was really crucial, it would become worth paying for, right?
I think with advances in AI-assisted decompilation, we may soon end up in the situation where given a binary you can produce realistic-looking source (sane variable and function names, comments even) which compiles to the same binary, even though non-identical to the original source code
There was a story where Microsoft patched a program for which they likely lost the source: https://www.bleepingcomputer.com/news/microsoft/microsoft-ap...