This is exactly the point. Fewer truly independent code bases, increased distribution of knowledge of the code, more tools deployed for automated verification/validation of the code, etc.

Proprietary code usually ends up devolving to the point most of it is treated like a black box. This is because knowledge of the internal code is restricted. And then over time the institutional knowledge is lost as people quit the effort (nobody is immortal).

At least with open source, knowledge can be distributed over larger bodies of people, and more experts can exist for the entire ecosystem to leverage. For applications where no "secret code" or "secret sauce" is required and in fact is nothing but a danger, Open Source is the solution.