Greek, yes (owning to their paganism, which had trouble dealing with existence; it is not possible to conceptualize existence), but we see very deep insights in

Greek, yes (owning to their paganism, which had trouble dealing with existence; it is not possible to conceptualize existence), but we see very deep insights in the medieval period, most notably in Thomas Aquinas. In the 20th century, we saw a particular brand of Thomism called existential Thomism that emphasized and expounded on Thomas' understanding of existence to help better appreciate it. But I am not certain that is your criticism, or that this applies here? Platonism posits a third realm of ideas, one which Aristotle denies and attacks (and which Plato acknowledged), instead locating universals in things and in the intellect that grasps them. However, you can restore a kind of realm of ideas by interpreting it as (the mind of) God. So, universals exist in all three cases, though the manner in which they exist varies (e.g., in things as concrete instantiations, vs. in the human intellect as concepts abstracted from the concrete).