I will start off with saying I feel the Halo 5 sales are a disappointment obviously and it proves consumer loyalty and trust is a very needed thing. However... I feel Halo 5 is rebuilding that trust in the Halo community with its multi-player for sure. Halo 5 sales will be lower in November, but I bet it still does 500-700k due to Black Friday + free games w/ consoles, etc.
Halo 5 will more than likely keep selling long term (though in much smaller #'s obviously) and I still think it will end up with 5+ million in the end. Nothing compared to past mainline Halo's, but still really good. It just is not the pure juggernaut it use to be.
Though I think with Halo 6 it will be up over 5, but below 3/4 again. Halo 4 and MCC really hurt the trust/loyalty of the fanbase and just like MS as a whole has been with XB1, they are having to do the same with Halo and I think Halo 5 is a step in the right direction.
I also feel this way about AC: Syndicate, Unity clearly hurt them bad, but I think Syndicate being as good as it is will help the next games sales.
Just my 2 cents, feel free to disagree as it's just my opinion