I think this is a very relevant point. I think (correct me if I’m wrong) the effectiveness of the best intervention in the world >>> the effectiveness of the best intervention in a random country X. So, it would be more beneficial to have 100 donors for effective global issues compared to 500 donors for effective national issues.
A caveat, however is value promotion. This is difficult to measure or quantify. There is a chance of large spillover effects due more people having an “effective giving” mindset. These people may further spread the idea of effective giving, or may become globally-aligned in the future. Off the top of my head, I think the spillover effects would be rather modest, but we’d probably need more “hard evidence” for this argument.