Systematicity not enough to argue for classicism
Even if classicism provides the best account of systematicity, it doesn't follow that the mind has a classical architecture. Systematicity is just one of many phenomena that the theory of mind must explain (eg perception, imagination, emotion, etc).
To show that the mind has a classical architecture, then, the classicist must demonstrate that classicism does better than connectionism—or any other competing theory—accounting for all of the relevant phenomena, not just systematicity.
Michael Anthony, 1991.