I explained the reason for that rapid decline for GT6.  Allow me to repeat.  GT6 is the first one to release after the launch of a new console on the old one.  That is obviously going to effect its mindshare and significantly hurt sales.  Also the second game on a gen usually has a decline for GT and the PS3 had the lowest install base any second installment had faced.  GT4 had over 100m install base when it released and growing pretty rapidly still and it still declined 3m.  I agree that one installment should not effect a series' overall credibility as a 10m seller but for Halo the one that DID sell 10m is the exception rather than the rule while GT is the opposite as even GT2 almost reached 10m.

So Xbox One won't be past 8 million consoles sold at the end of 2015? Besides 10 million is lifetime so even then another two years will be needed before we see what Halo 5's Lifetime sales will settle around at.  So by 2017. Like I said I think it'll end up around 8-9 million. 

I really dont think many games will reach 10million sales anymore.

Maybe if it has a 100% attach rate and everybody buys 2 copies..

But on a less jokingly and more serious response.

I predict it'll sit around 5.5-6m lifetime when all is said and done.

First Halo is not declining, it is just that the multi is Halo 4 was great considering it was the first ever game by 343 but it lacked the true Halo in it. We don't even now the sale numbers for Halo 4 VGC says 9m above. Ms announced that Halo 4 has outsold Halo 3 every month comparing the launch year moreover you need to know Halo 3 had more months to sell but Halo 4 still outsold it. I can safely say the game is somewhere near 10m, ofcourse digital sales cannot be 1m but the discounts offered on the game digitally were insane.

The Halo beta will tell us how much the game will sell if it is the true Halo I can see that game doing more than 10m. From what we have seen Halo 5 beta looks great.
So your statement is wrong. Sorry to say