While I am a Nintendo fan and not an Xbox person, I don't think Xbox fans should be too down on Halo 5's sales. Every franchise goes through the phase where it needs new ideas and I am sure that Halo 6 will get a big makeover because of the poor sales of Halo 5. It would be unnatural and unhealthy for the Halo series if every entry sold insane copies even if the storytelling and gameplay is getting stale. Poor sales are a wake-up call for the developer to bring in new ideas and/or go back to a series' roots and I am sure a company like Microsoft is going to be doing a little of both with the next installment.
Take Super Mario Sunshine, as an example. That game was a bit of a letdown for us Nintendo fans and I think it would have been reasonable for fans to speculate back in 2002 that the 3D Mario platforming series was on its way out. It turned out to be the contrary, though, because Nintendo took the failure that was Sunshine and made Super Mario Galaxy which is one of the greatest Mario games ever made. I suspect the story will be similar for Halo, people just need to relax and give it time.