I thank Halo V will continue to sell well and through the years. However, both recent Halo launches: Master Chief Collection and Halo V have suffer from launch issues. I don't think Microsoft will allow that to happen again. They should be fairly familiar with the system and DirectX 12 by now.
I think bad word of mouth at launch times has hurt Halo (and Assassin's Creed). It kills the buzz, and I think results in a say 20-30% decrease in sales for the week, which translates in to significantly overall loss in sales.
So I think Gears of War 4 may have a better launch than Halo - as I think it will have an amazing launch. The Beta feedback has been good too. But in the long run Halo will do better as it is just a bigger world and a much more involved story (with books, movies, etc.)
Microsoft needs a Wow! fall launch, and this is a big chance. I