Pessimism about motivating modal personism

In 'What's Wrong with Speciesism?', Shelly Kagan sketches an account on which both actually being a person and possibly being a person are relevant to one's moral status, labelling this view 'modal personism' and supporting its conclusions with appeals to intuitions about a range of marginal cases. I tender a pessimistic response to Kagan's concern about motivating modal personism: that is, of being able to 'go beyond the mere appeal to brute intuition, eventually offering an account of why modal personhood should matter in the ways we may intuitively think that it does.'