but that a priori we should assume diminishing returns in the overall spending, otherwise the government would fund the philanthropic interventions.
I think this is fundamentally the crux—many of the most valuable philanthropic actions in domains with large government spending will likely be about challenging / advising / informationally lobbying the government in a way that governments cannot self-fund.
Indeed, when additional government funding does not reduce risk (does not reduce the importance of the problem) but is affectable, there can probably be cases where you should get more excited about philanthropic funding to leverage as public funding increases.
I think this is fundamentally the crux—many of the most valuable philanthropic actions in domains with large government spending will likely be about challenging / advising / informationally lobbying the government in a way that governments cannot self-fund.
Indeed, when additional government funding does not reduce risk (does not reduce the importance of the problem) but is affectable, there can probably be cases where you should get more excited about philanthropic funding to leverage as public funding increases.