That's definitely one important aspect. And even stuff not formally fossilized often becomes de-facto fossilized due to the way funding for development works. Things are often very bursty: a large library or piece of software may be written over a period of 2-5 years of concentrated effort either by a PhD student, or by programmers/research-scientists/post-docs hired on an NSF/DARPA/EU-funded research project. But then the PhD student graduates, or the project ends (and therefore its funding for programmers), and the software goes into much lower-staffing maintenance mode. In that mode there aren't resources available for anything but minor fixes. There are some very high-profile projects that are an exception to that pattern, because they're seen as important enough that they manage to string together continuous development for years or decades, either through a series of PhD students or a series of grants. But lots are more or less write-and-then-maintain. Despite being lightly maintained, if the initial work was solid and produced a reasonably "complete" output, it might still be useful to other researchers for years into the future, if it doesn't bitrot. Some of the R packages are a good example: plenty of stuff hasn't been touched in 10+ years but is still in daily use.
