Halo 5 probably won't sell as many for a few reasons. First, when it releases it's unlikely that the Xbox One will have sold as many units as the Xbox 360 had 2 years after its launch which is when Halo 3 released. So that alone will make for a hurdle to the sales. Second, the FPS space is going to be much tougher this time than it was when Halo 3 launched. Hell it will probably be much tougher than it was for all of the last generation. With Call of Duty still being a big deal, Battlefield making a name for itself on consoles, Titanfall releasing earlier in the year, and Destiny launching a couple months prior (assuming a Holiday 2014 release) the game is going to have its hands full trying to pull people away.

If 343i doesn't try to really do much new with the game and people feel that the franchise's gameplay has become stagnant that will also affect sales. Of course that's just a "what if" reason. Something that I have noticed though is just a growing lack of interest in the universe of Halo. It's still big. There should be no doubt there. I just don't get the same sense of excitement from people over it like when Halo 2 or Halo 3 came out. By the time Halo 4 came out the hysteria surrounding the franchise was not quite what it was about 5 years before. It will likely still sell 5 million copies or more when it's all said and done. I just wouldn't expect another 9 million or more sales unless Halo 5 blows people away.

Actually it's not, it's just because it hasn't been out as long. Halo is position to sell more than Halo: Reach, close to what Halo 3 sold. Halo games tend to have long legs. In addition, Halo: Reach was a very different game to play than Halo 1,2,3. In Reach you play along side a team. Master Chief rarely has help survive long in Halo. :D

Halo is still HUGE. It has a great lore and story behind it too. I think it will last as least as long as Star Trek or Star Wars - and they are both still going strong. Halo is still different enough from them to be interesting. And while we do know much about the story, there are plenty unknown secrets of the characters. There is much more to explore.

TL/DR: Halo 4 will probably outsell Halo: Reach, but not as much as Halo 3

I really doubt it for two reasons. The first is that the online communities right now on Halo Reach and 4 are tiny compared to what they used to be for years on 2 and 3. People have lost interest faster than ever. The second is that even though Halo 4's numbers look really great right now, it dropped down to $30 standard retail faster than any other Halo game, ever, plus lots of stores have been selling it for as low as $15 brand new. I remember 1,2,3, and Reach all being expensive for a while. The only way that it will do over 10 million is if they truly innovate and make something that feels and looks fresh. People pick on the number of Zelda and Mario games Nintendo has made but almost all of them feel fresh and new compared to previous entries, which is why they still consistently sell well.

Mario games rarely feel fresh and new, I've played enough of them to know that. Recent Zelda games haven't been the best selling in the series. Also you say those series still sell well. Have past Halo games games bombed? I'm pretty certain Halo 4 did extremely well.

During Black Friday I did not see any stores selling Halo 4 for $15. But I did see some retailers selling GTAV for $20. Hence, your example proves nothing.

I dread what they'll do with 5 to be honest, if they do a repeat of Halo2 with the pc version being Vista exclusive I foresee them holding people to ransom for either always online DRM or some new form of paywall which hasn't been considered yet, Subscription based online akin to an MMO?

Altho you never know, if the X1 was to slow down a bit in the new year in terms of hardware sales they could consider a port to the 360 as well as releasing the Next gen version of H5 considering Microsoft have said they're going to continue to support the 360 for some years to come.

i thought you ment quality, or enjoyment of the franchise and was about to chime in that i felt campaign has gotten better since the low point for me of halo 3 (i loved odst) felt reach was enjoyable and 4 was pretty good.

in terms of sales depends on when it comes out, i could see a massive attach rate for the console base if it comes out next year but even if it gets 50% attach rate due to the low number of ones vs 360s will probably sell less. unless it is 360 and xb1. I doubt it will be though.

side note i would really like halo 2 to come out on the 360 like the original halo did so at least for a short period all mainline halo games would be on one console, two was also the one i spent the most time in multiplayer on

Well we know for sure Halo: Reach passed 10 million including digital sales, it was bundled with Fable 3 and that got a 1m+ boost due to the bundle, Halo: Reach must be around 11 million by now including digital.

Halo 4 is still selling well, will probably reach 10m at retail eventually and well over that including digital.

There may be a slight downward trend but it's nothing huge, it's pretty insignificant to be honest. I expect Halo 5 to sell in line with Reach, I doubt 3 will ever be matched simply because it was on sale for so long, released before the rise of Call of Duty and the Xbox 360 did so well.

I think ur kinda exaggerating the decline, Reach is a prequel and u dont even play as Master Chief so its understandable that its sales are lower than Halo 3. As for Halo 4, its tracking pretty similar to Halo 3 and will most likely pass 10m despite next gen cutting it legs short.

No, but I hope that will be as good as 4.