As we have seen, Bobonich’s Claim has an all-or-nothing character. He says: according to the Phaedo and Republic, no non-philosophers can be virtuous or happy; according to the Laws, all citizens, even non-philosophers, can be virtuous. That's a chief virtue of his Claim: it's a sharp thesis.
But I wonder if this is the language of Platonism.
What I mean is, Platonism is a philosophy of ideals, and of departures in different degrees from those ideals. A Platonist thinks that there are Forms, and then particulars which participate to varying degrees in those Forms. The Divided Line, the Ladder of Eros, and the Cave Allegory are all devices to represent degrees, and stages, of approximation to some ideal. Nothing is all-or-nothing, on this way of looking at things, except the distinction between the ideal and everything else.
Suppose virtue itself follows this pattern. Then ideal virtue, ‘true virtue’, will be something the gods have, and which a philosopher might just attain, if he gets everything right, but which everyone else will have to a greater or lesser degree, depending upon his approximation to the ideal.
It would be easy to hold, on this picture, both that “All non-philosophers are non-virtuous”, that is, that non-philosophers can only approximate to virtue, and that “All non-philosophers can be virtuous”, that is, that non-philosophers can approximate to virtue.
But Plato clearly does look at virtue in this way. Recall Symposium 212a: “’Do you think it would be a poor life for a human being to look there and to behold [Beauty itself] by that which he ought, and to be with it? Or haven’t you remembered,’ [Diotima] said, ‘that in that life alone, when he looks at Beauty in the only way that Beauty can be seen—only then will it become possible for him to give birth not to images of virtue (because he’s in touch with no images), but to true virtue (because he is in touch with true Beauty). The love of the gods belongs to anyone who has given birth to true virtue and nourished it, and if any human being could become immortal, it would be he.”
I don't know if Bobonich considers this objection in his book; I need to look into that. (But the Index indicates he does not discuss Symposium 212a.)