The problem with your argument is that what you call agent is nothing like what Minsky envisioned. The agents in Minsky's world are very simple rule based entities ("nothing more than a few switches") that are composed in vast hierarchies. The argument Minsky is making is that if you compose enough simple agents in a smart way, an intelligence will emerge. What we use today as agents is nothing like that, each agents itself is considered intelligent (directly opposing Minsky's vision "none of our agents is intelligent"), while organized along very simple principles.