In addition, even for AAA games (but also for other games and software, this adds extra dependencies that may be broken outside your control. At a company i worked several years ago, we spent more effort upgrading Visual Studio because for some middle we only had DLLs (and their libraries) to link against and had to obtain new versions (part of the effort was that the company that wrote said middleware hadn't upgraded their own so we also became QA for their compatibility with the newer VS, though i don't remember if there were any issues in practice).
Of course having the code doesn't mean updates will be frictionless, but there will certainly be way less friction and you wont need to wait for someone else.
(btw i think Visual Studio has tried to be backwards compatible in recent versions with binary C++ libraries, but i don't think this is something you can rely on in the long term)
Also all that are with the assumption that the requirement is that your code remain on the same platform and targets. But at some point you may want to work with another platform (be it as host or target) and not having source code can make that from incredibly difficult to impossible. This may not seem much of an issue for something as platform-specific as DXIL - but actually notice the article mentioning that the binary blobbiness of DXIL made it impossible to precompile shaders on platforms outside specific Windows and Linux architectures for which the DLL was provided by Microsoft.