By Halo standards Halo 5's sales were bad, but by overall AAA standards they were pretty good. Halo was always an anomaly amongst big-budget AAA games. Halo 2 blew away previous record-holders for launch month sales, including multiplatform titles. In October 2004, GTA: San Andreas, the biggest game of 2004, debuted to about 2,054,000 copies, and it sold another 1,536,000 in November, for a total of 3.59M over a ~5 week period. Meanwhile, Halo 2 sold 3,267,000 copies that November alone; it released on the 9th, and I think the NPD November sales period ran from the 1st to the 28th (Nov. is always a 4-week sales month), so that's like 20 days of sales for Halo 2. Halo 2 did drop a bit in December, selling nearly 936k vs. 1554k for San Andreas.

While GTA:SA was ultimately the bigger game in lifetime sales, Halo 2's launch sales set records that wouldn't be beaten until Halo 3 came out three years later. Halo had huge launches. That was its thing last decade. Other major games rarely broke the 2 million mark during launch month until COD started to become popular. Even to this day, one or two million copies at launch is pretty much par for course for popular AAA games, with a select few multiplatform titles selling more copies. Including bundled copies, this past November Black Ops III sold about 4.89M at retail in the U.S. across all platforms, while Fallout 4 sold about 2.54M and Battlefront sold 2.1M. Those were all big multiplatform games. As far as exclusives go, nothing came close to touching Halo when it was in its prime. One million copies is a great launch, but 3 million is spectacular. There's a reason Halo 2 & 3's launches were newsworthy. They were an anomaly. Games just didn't have launches that big at the time, and even then 3+ million on launch month still remains a remarkable and very rare milestone for a game to achieve on a single platform. With COD having passed its peak, we might not see a game sell more than 3 million on a single platform during launch month until GTA6 is released (GTA5 sold over 4 million on the 360 alone in Sept. 2013).

Having launch month sales in the 900k to 1M range should still sustain Halo, but it's obvious that the series has lost a bunch of customers and that we won't be seeing the huge launches we used to see with the series. Such freakishly large launches were an anomaly, and still are to a degree, and perhaps because of various factors not as many people can get excited enough about Halo anymore for the series to pull 3M or even 2M at launch.