Thomistic Moderate Realism Reduces to Armstrongianism or Platonism

I've always had difficulty understanding Aquinas's "moderate realist" view on universals, at least as that view is expounded by his interpreters. It seems that they want to have their cake and eat it too: Thomists both believes that universals are "objective" and "extramental" in some sense. But they also are not extreme realists, i.e., (modern) Platonists. They also seem to say things that make it sound as if universals are merely conceptions; if taken at face value, that reduces to conceptualism or nominalism of some sort.

Here's one way of bringing out the problem.

Assuming there are universals, then:

• 1. Either (a) universals exist extramentally or (b) they do not exist extramentally.
• 2. If (b), then nominalism or conceptualism, QED.
• 3. If (a), then either (i) they only exist in the objects that have them, or (ii) they sometimes exist outside of the objects that have them.
• 4. If (i), then that is Armstrong's view.
• 5. If (ii) then that is Platonism.
• 6. So if (a) is true, then either Armstrong's view or Platonism is true, and so moderate realism either reduces to Armstrong's view or Platonism.

6 already seems to disambiguate the Thomistic view in a way that makes it unacceptable, and does not let it have all of the desirable qualities it is supposed to have.

• 7. If Armstrong's view holds, then universals depend on the objects that have them, and therefore cease to exist if the objects do.
• 8. But if the universals cease to exist, then statements about non-existent things, like "dinosaurs are big creatures", do not require universals to be true, and so universals are superfluous.
• 9. So if Armstrong's view holds, then universals are superfluous.

That only leaves Platonism, and there are huge problems with Platonism.

To be fair, I am leaving much unsaid here, and I am not making any distinctions within "Platonism." But ultimately I think this is basically correct; the way Platonism is construed in modern times basically just is the view that there are non-mental, objective, necessarily existent, universals.