 I think that there are at least three problems with this argument. The first is that the parsimony comparison between the nihilist and existence monist inventory is problematic, since the ontologies in question are disjoint. This is not a case where one ontology is a proper subset of another (rather both are disjoint proper subsets of the universalist inventory). Rather each inventory is itself minimally complete. From the perspective of each inventory, there are no elements that are superfluous.