For open source code, I think the concern is practical: bit rot is real, and libraries with no recent updates are more likely to have suffered bit rot. It's not that a library can't be finished, but that the world in which it lives never stops moving. It can be a bigger or smaller problem depending on the ecosystem. Old C89 libraries have a good chance of still working, because the ecosystem isn't moving so much any more. Old C# libraries almost definitely don't, because the ecosystem is moving quickly.