I think you answered your own question? The index fund would just allocate in proportion to current donations, reducing both overhead for fund managers and the necessity to trust the managers’ judgement (other than for deciding which charities do/don’t qualify to begin with). I’d imagine the value of the index fund might increase as EA grows and the number of manager-directed funds increases (as many individual donors wouldn’t know which direct fund to give to, and the index fund would track donations as a whole, including to direct funds).
I think you answered your own question? The index fund would just allocate in proportion to current donations, reducing both overhead for fund managers and the necessity to trust the managers’ judgement (other than for deciding which charities do/don’t qualify to begin with). I’d imagine the value of the index fund might increase as EA grows and the number of manager-directed funds increases (as many individual donors wouldn’t know which direct fund to give to, and the index fund would track donations as a whole, including to direct funds).