Why only a focus on Open Source? I feel like vulnerabilities in closed source products like Microsoft Office, Microsoft Windows, and Google Chrome to name a few can be just as essentially and foundational as other open source software for many businesses.
I think the idea is that automated source code processing is making it possible to find vulnerabilities at great speed and in an overwhelming way in software that does not have paid maintainers, whereas closed source software in active use has both less accessible code and paid maintainers.
A charitable foundation might be plausible to help companies secure their closed for-profit software but it doesn’t really have the same urgency for the fabric of the internet (or the same moral clarity)