Medium-sized donors can often find opportunities that aren’t practical for the largest donors to exploit – the ecosystem needs a mixture of ‘angel’ donors to compliment the ‘VCs’ like Open Philanthropy. Open Philanthropy isn’t covering many of the problem areas listed here and often can’t pursue small individual grants.
This reminded me of the following post, which may be of interest to some readers: Risk-neutral donors should plan to make bets at the margin at least as well as giga-donors in expectation