The decline began with ODST and Reach, both by Bungie so don't go thinking that it's all 343i's fault.

Gameplay wise it is up there with the best but my problem with 5 is that I just didn't like Locke as a character.

And yet sold shity tons copies... why? It is a success no matter how bad you think it is... the most sussceful new IP this gen and the expectation for Destiny 2 are to become the one of the big one sellers like CoD.

Review has nothing to do with that... Destiny took a lot of "fan" base from Halo.

Even after Halo 5 launch I saw a lot of gamers playing Desitny on Xbone... CoD, Fallout and Battlefront affected more the player activiity in Destiny than Halo 5 and at reset days they are coming back yet... a lot of player on Xbone back this week to play Iron Banner for example.

Something Destiny is doing right, no?

And yes I believe it took a lot of Halo fanbase mostly due Bungie... people like more Bungie than i343 for example... you can't left Destiny out in any discussion about "why Halo 5 sold bad compared with previous titles?"... Destiny is one key point... more than MCC issues, lack of split-screen, etc.

As a non Xbox user I would say that lost faith is the biggest reason.  Lost faith by a portion of the fanbase in Microsoft, the series itself, the developer, certainly not the hardcore fans of which there are many.  Also the surrounding releases of Fallout 4 and COD. It may well be a slow burner and people should enjoy it anyway, it didn't sell that badly and has some good feedback too.

Franchise fatigue
Destiny
360 owners buying PS4s
Halo MCC
Lack of splitscreen

Imagine how bad it could have been if people knew that the advertisements were a lie before the release.

Let´s brake it down:

1 - With $500mi investiment it would sell no matter what. TTK numbers should make you woried, it barely reach top 5 in September, a very slow month.

2 - It is cross gen.

3 - Well, this one would deserve a thread of its own. But I will try to be short. Are you saying people that complained about Halo 4 campain jumped to Destiny? Are you saying that those guys for that Reach sprint and jet-pack killed Halo balance? The fan base Destiny took from Halo is the casual and young players. Or people that get easely adicted to loot and shine things. 

4 - Anedocte. But... do you have a XBone to keep track of what your friends are doing?

5 - Destiny is a online game that don´t even need a large population to survive. People can play the endless cycle of griding and reward and never experience PvP. It just need a couple of friends. That will likely be as hooked than you.

6 - You clearely haven´t played any Halo game, and might be living under a rock the last year and a half if you belive that. Bungie was under presure since Halo 2, which in my opinion has a far worst campain than Halo 4 or 5. When I first played Halo 3 I had no F*%$ idea what was going on. Reach, which I think is the best overall game, suffered a shitty storm because they changed things.

That´s it. 

Its not a very accessible game. Period.

Long time Halo fans should love it. But people coming onto Halo expecting some casual / social fun will be treated with pitchforks. Its almost as if 343 want the noobs to go back to playing COD / Destiny.

1. To know the story, you need to complete Halo 4, Spartan Ops and read a couple of books and watch the TV show. Its a very tall order and I found myself turning to wiki to find out what was going on.

2. The multiplayer is phenomenal. But by making everything ranked, if you come onto Halo 5 to expect some social fun, you will get your ass handed to you plain and simple.

3. Lack of modes at launch. Not having big team at launch is stupid Making big team forge maps is twice as stupid. No Forge. No party games. Specified maps for specified game modes is also stupid. 3 maps for warzone is pathetic.

343i / MS may as well just go ahead and call this game a spin off and rework for Halo 6.

1. Other big titles
2. MCC issues
3. People endlessly complaining about everything

1. TTK sold amazing good...  it is a expansion that shows gamers wants more Destiny...

September NPD (bundles included): 713k
October NPD (bundles included): a bit over 200k

That only new players... you know TTK is a Expansion and not a new game... if you have Desitny why you will buy other disc?

Are you sure about what you wrote here?

2. Yes I beleive Bungies fans follwed Destiny instead Halo even on Xbone... and no matter if it is casual or youngers (not true because I play with die hard old Halo fans on Destiny).

3. Just look at your friend list to see people are playing Destiny (I'm sure if you have a Halo friend list you will find a lot of them playing Destiny)... and yes it is anedoctal.

4. No matter how the game is people are playing and liking it... btw the activies are well divided because you can play only PVP too... Bungie give us stats about what people play in Destiny and PVE and PVP are pretty close to each other... Iron Banner this week make the online play list back to old Destiny days before CoD, Fallout and Battlefront launch.

5. I played all Halo games included the last one called Halo 5... and I have different views of the quality of the old titles... Halo 2 campaign is way better than 4 or 5 for me... 3 is the best overall game and ODST is an amazing expansion... Reach have the best PVP modes to be fair.

Well I guess we disagree a lot.

But it is a no brainer Destiny took a lot of users from Halo even on Xbone.

1. Being an exclusive on Xbox One just isn't the same as being an exclusive on Xbox or Xbox 360.

2. Making a game that satisfies an extremely diverse community (that gets more fanatic and diverse after every installment) while still being accessible to newcomers is a hard job, especially when you have strong competition.

3. Halo sales stopped growing after Halo 3 even though the 360 was selling increasingly well.

Taking that into consideration, Halo 5 sales are actually not that surprising.