So going from Halo 3 (and eventually 4) there was a HUGE dropoff in sales and popularity. Why do you think that is?

Here's my opinion on the matter... they changed too much. Small changes here and there are alright so long as they further the development of the series, but at a certain point, you might as well just make a different game. The changes with Halo 4 were cringeworthy but definitely forgivable. When Halo 5 was coming out they announced that they'd listened to the fans and were going back to the roots of the originals, then they went even further out -_-

Story / Characters

Locke is a horrible character. They should have given his story to Arbiter. It would have been way more engaging considering the history. Also there is WAY too much focus on peripheral storytelling. The story became too complicated. Hunt the Truth, the comics, the DLC it's too much. You shouldn't need that to play this game. Character split should have been MC 50% and Arbiter 50%. If they wanted to introduce Locke, he should've gotten the short end of the stick, and they should have made his character a lot more interesting and conflicted like the Arbiter was. Also the fact that the Elites are villains again after Halo 4 is stupid. It undermines Halo 3.

Aesthetics

The architecture of the game worlds is too over the top. Look at the aesthetics of Halo 1-3. There are alien accents to more simplistic designs. After Reach and Halo 4 and now Halo 5 they have made things look more and more intricate to the point where the conceptual aesthetics are completely different from the original trilogy. The way armour looks in Halo 4 and 5 is way too different from the originals. It all looks plastic compared to the perfect design in Halo 3. And that H5 scope? Barf.

Guns

The guns look way too different (see shotgun and rocket launcher especially), and the lack of dual wielding and the inclusion of the DMR are insulting. The whole reason Bungie did this in Reach was because in Halo 1 you don't have dual wielding, so if you wanted to do a chronological playthrough, it made sense to exclude dual wielding in Reach. The inclusion of the DMR in Reach was due to the fact that the BR hadn't been invented yet (in universe) so having a prototype to the BR was an excuse to have the same type of gun in Reach... so then why is the prototype back in Halo 4? They nerfed the BR to make it different than the DMR and OPd the AR in Halo 4 and 5 to give noobs a better chance against the BR/DMR.

Music and Sounds

It doesn't SOUND like Halo. The music is way too different as are the sound effects. We have hints of the original Halo music in Halo 5, but barely anything. Halo 4 didn't have any at all! And I know people will argue that they were trying to update the SFX of the guns etc, but it's not that it sounds crisper, it sounds completely different. The DMR in Halo 4/5 sounds more like a progression of the BR from Halo 3 than the BR in Halo 4/5 does.

Multiplayer

The maps in Halo 4 and 5 are GARBAGE. Halo 1-3 had some of the best maps I've ever played in any FPS. They were straight forward, simple, sleek and you only needed a couple minutes to learn the map. Halo 4 and 5 the aesthetics became way too busy, all of the maps look the same and they have no originality or character to them. Also the lack of local multiplayer was some real bullshit. Halo was that one game that you'd get together at a friends place to play - those days are apparently over.

Gameplay

Some of the armor abilities are welcomed, others aren't. The way they handled abilities in Halo 3 was amazing. You pick them up as equipment and you get to use it once. If you don't, someone can kill you and take it. If they included the likes of hologram and jetpack with bubble shield, degen, regen, flare, mine, active camo and overshield that would have been amazing. After replaying all of the halo games recently, I'm down to remove sprint. It's not necessary. They're Spartans - they're always running. If I want sprint mechanics I'll play CoD.

Changing a couple of things here and there is one thing, but changing as much as they did is downright stupid. Switch all of the above back for Halo 6 and all of those old players will come running back.