The question of whether they legally or illegally monopolized the debit card market is sort of besides the point.
This should be a publicly held function, just as printing money was a publicly held function. Visa shouldn't just get to take a 1% cut of GDP (or the consumer portion of GDP) forever because the market is a natural monopoly.
Updated post today:
DOJ accuses Visa of debit network monopoly, impacts price of 'nearly everything'