You are extremely extremely wrong.
Days first day sales in just Japan was 291,000 copies, and it total sold 571,981 units in Japan alone. It has also sold 360,000 units in North America as of October 2009, meaning it has surpassed 500,000 in America as well. No data is available in Europe, but it should be trending just a little bit lower in Europe, meaning Days has sold about 1.5 Million worldwide.
If you use ratios and hope for the best, if BBS sold 468,000 units in Japan, that likely means it will hit around 900,000 units in Japan, on track with Crisis Core and Dissidia, so its definitly in the correct realm of possibility. In the states however, there is no telling how it will fare. While Crisis Core sold over 500,000 in both North America and Europe, Dissidia only sold 200,000 units in America, but you can thank a year long delay and a huge amout of piracy for that (Square Enix should not repeat the delay mistake again, why wait a year when you can pirate the Japanese version).
All in all, if we take into account piracy, Birth by Sleep will outsell Days, but likely just barely hit two million. If my piracy fear does not exist, it can easily surpass 2.5 million.