It's an interesting idea. The current endowment size of less than $1M is immaterial; the question with a project like this will always be how it is able to raise capital.

A way something like this could be interesting is if founders started donating 5% of equity when they started a company to an open source foundation like this one.

It doesn't impact the founder much financially: Success is very binary for founders. But in aggregate, if thousands of startup founders do this, there would be some hits and some of those hits could generate a significant endowment.

(You can also try to get people to donate who feel their success was built on top of open source, but I feel that after 10 years building a company to IPO, one's attention as a founder has likely been on business metrics and spending time with business people, not on technology and spending time with technologists, and that shift in attention can reduce people's feeling of gratitude for the amazing inheritance that is open-source software).