Platonists say that Platonic Forms are beings which exist. I do not think they do exist (and I don’t even think they are ‘beings’, contra the neo-meingonian). However, in thinking about Platonism, which has occupied me lately, it seems to me that even the Platonist won’t want to say that Platonic Forms exist independently of one another strictly (even if they seem to be saying that each platonic form’s existence is due to that particular form’s nature of necessity). ‘6’ for example, could not exist without the concepts of other numbers like ‘1’ or ‘2’, to which 6 is intrinsically related and inexorably bound up conceptually. The concept of ‘6’ seems to be composed of more basic ideas like ‘1’. Even if one shouldn’t say that ‘6’ is contingent upon ‘1’ or/and ‘2’, still one should say that they are inexorably bound up, so that if any one obtains all of them obtain. We can also imagine other platonic forms like being ‘smaller-than’ or ‘larger-than’, concepts which may require the concept of being ‘extended in a space’. Concepts are all theoretically bound up.
Perhaps, then, one should say, in preference to saying that Platonic Forms are all individual beings which ‘exist‘ independently of one another (each being necessary on their own grounds, even without having the property of aseity), that a Network of Necessary ideas exists, and this Network of ideas can be conceived of by loose analogy to the tree of life as drawn on the assumption of common ancestry (different domains can be called different species, and some platonic forms might bear a stronger family resemblance to others, but the idea is that they all bear some family resemblance). Perhaps the borders of that Network can be argued (maybe the Mathematical Platonist will want to exclude all things other than mathematical objects, though I can’t see how that could be done since logical laws are not mathematical objects, etc.), but regardless, there is some such Network of ideas. These ideas are conceptually bound up with one another at some level, none of them are independent of each other; one exists if and only if they all exist. This Network of ideas exists of necessity; a kind of family tree.
Obviously if that Network of ideas simply belonged to some necessary existent being then that would explain why we take these things to be necessary even though they lack having the property of aseity – and we can tidily account for the necessity of this network of ideas by appealing to one necessary being on whose nature the ideas supervene, that necessary being having aseity.