No development + no source + time = broken.

Yet. If it's "dead", it's only a matter of (not very much) time before it doesn't work anymore.

That software can't ever just be finished is a problem that needs to be solved.

This problem doesn't seem tractable in anything approaching the near term. There are just too many dynamic layers, each with their own competing product ecosystems, which in turn each are controlled by competing interests. Perhaps at some point in the future the entire stack from hardware to application level APIs will become commodity products that nobody benefits from differentiating. At that time, perhaps software can just be finished once it arrives at feature maturity. Until then, UX, architectural, security and other assumptions made about the application environment will continue to age poorly, and software products will become outmoded unless actively developed.

To an extent, having the source available is a good solution.