If all donors to charities A and B were identical to you, then your decision to donate $d to charity A would be equivalent to a decision for all donors' funds to go to charity A rather than charity B

You assume that either all decisions are made simultaneously or that rational donors are insensitive to diminishing marginal returns as they observe greater funding inflows to A, neither of which ought to be the case.

In particular it wasn't the case this winter when we observed sufficiently lopsided funding flows into MIRI vs. CFAR and told some of our donors that their next marginal dollar ought to go to CFAR until their fundraising drive closed.

That said, I agree with a lot of what Holden said about this recently - does anyone have a link? - in which he pointed out that it would be unfortunate to have many 'rational' donors effectively trying to cancel out each others' allocation splits, and I don't actually object to Holden's commonsense approach (quoted below) about "allocating your dollars in the same way that you would ideally like to see the broader GiveWell community allocate its dollars". Or you could mix the two approaches and give some money in a way that matches what you think should be the overall distribution, and the rest to whichever charity you think is most neglected provided it is severely enough neglected.

I wouldn't mind seeing a formal analysis of why agents with certain types of noise of them would end up more robust if they didn't donate all to one charity. Maybe this would arise if we suppose that lots of people are in reality mostly insensitive to diminishing marginal utility and don't compute it very well or very exactly when splitting between charities that all already have "some funding".