You are making a circular argument. For a universal to be real does not imply that it's a concrete. That's only true under the premises of a non-realist metaphysics. You are assuming a metaphysics in which only concretes are real, and then telling me abstractions therefore cannot be real because only concretes are real. But it's your premise that I'm disagreeing with in the first place.
The distinction between "abstract" and "concrete" is whether some thing is universal or specific - not whether the thing exists or not.
The question of the reality of universals is a question of whether there are metaphysical natures, whether there are such abstract "kinds" in reality, or whether everything in reality is purely a specific and concrete, not of any real class or kind (other than those subjectively invented and justified).
(side note, when someone uses the phrase "existed metaphysically", what they really mean is "existed physically" - they don't really understand the term "metaphysical". The "metaphysical" is not another realm of existence out there in the heavens above the physical. There is one realm of existence: the universe. Metaphysics is the study of the nature of things, and whether things have such a "nature". If a given thing has a metaphysical nature that means it is of a kind (a "kind" meaning a type or class of things). The thing itself is the concrete: it is entirely a specific particular. The type or the kind of which the concrete is an instance or examplar is the abstract: it is a universal, and stands for an unlimited number and variety of possible instances or examplars. The only question of what "exists metaphysically", are exactly these universals, that's what metaphysics is. Hence such arguments from materialists and positivists and nihilists, et al., that "metaphysics" is a dead subject.)