At its peak, the Roman Empire held up to 130 million people over a span of million square miles. How could such a powerful empire collapse? Simple, the real effects of debasement took time to materialize... and then exploded.
The barbarian tribes who carved up the old empire — the Franks, Visigoths, Ostrogoths, the Vandals, and so forth — were much more Romanized than the tribes that had menaced Rome centuries earlier. The rulers of these new kingdoms generally sought to co-opt Roman elites that still held significant wealth and power across the former Western Empire.