I would guess a marginal donation usually effectively goes to one of the charities they would have funded anyway (in one period or another), not necessarily the first charity they would have not funded if they had less to donate, and sometimes to a charity they wouldn’t have funded otherwise, which would be less attractive than the ones they would have funded anyway. You can think in terms of shifting the distributions of charities and payouts if you want to avoid butterfly effects.
You might not expect decreasing marginal returns, in case the charities themselves have increasing marginal returns. You would expect decreasing marginal returns when your donation goes to a marginal charity or if the charities have decreasing marginal returns.
Then it becomes a matter of which charities and what those charities do with marginal donations.
I would guess a marginal donation usually effectively goes to one of the charities they would have funded anyway (in one period or another), not necessarily the first charity they would have not funded if they had less to donate, and sometimes to a charity they wouldn’t have funded otherwise, which would be less attractive than the ones they would have funded anyway. You can think in terms of shifting the distributions of charities and payouts if you want to avoid butterfly effects.
You might not expect decreasing marginal returns, in case the charities themselves have increasing marginal returns. You would expect decreasing marginal returns when your donation goes to a marginal charity or if the charities have decreasing marginal returns.
Then it becomes a matter of which charities and what those charities do with marginal donations.