[1/*] If we take the chance that a tool (compiler, linker, batch, whatever) remains working for a particular codebase after one year as a given probability p, then the chance that build remains working after x years is p^xn, where n is the number of tools used in the build.
It's much more complicated to do correctly - I suspect that "dependency interdependency" is a significant variable as well (how much your deps depend on each other), that would accelerate failure.