Final Fantasy XIII was at 5.55M worldwide as of March 31, 2010 (about a month after the US/EU launch). Sales were updated to around 6M at the end of the year, and again to 6.6M by the end of 2012. Games that were massively overshipped never get subsequent shipments, so Final Fantasy XIII couldn't have been too bad, as you said.
And FF15 is a much meatier game that will get even more content via DLC so it should see nice sales even into the following year. FF13 was heavily front loaded due to word of mouth being negative. 15 could be looking at a whole different trajectory.