Traditional Halo died with Halo 2. Why they bothered to add recharging health when recharging shields was already enough is beyond me. Now you can just duck behind cover and wait for your health to recharge, so taking risks and pushing the enemy has no downside if you survive the encounter. In CE you were punished for letting your health go down during an encounter so you would be weaker for the next one. That's one layer of strategy gone.
Traditional Halo died with Halo 3. Now instead of having to use your skill and dexterity to survive if someone gets the jump on you, you can just drop a bubble shield and make your attacker come in for close quarters. And there's no visual que to see what kind of equipment others are carrying so there's no way to strategize or plan around your enemy having a regenerator or what have you.
Traditional Halo died with Reach. Loadouts cause uneven starts, which make balancing maps for early encounters more difficult. The jetpack has a visual cue at least, but now you can just use armor lock as a get out of jail free card. This makes it much harder to read encounters than even in Halo 3.
All of these are legitimate complaints (Reach bringing back health was a brilliant move IMO), and they beg the question: where do you draw the line between "traditional" Halo and "non-traditional" Halo? Surely you can't just divide it between Reach and 4 because a lot of 4's DNA was sown in 3 and Reach. But Halo CE's multiplayer is about as different from 2/3 as Reach is, IMO. So was it Halo 2 that did it in? What does it mean that "traditional Halo is dead"? Is it necessarily a bad thing? Would it be preferable if Reach, 4, 5, and Infinite were just variations on 3? Or is there something to be said for experimentation and evolution?
Personally, I think each game introduced some good ideas (dual-wielding in 2, detachable heavy weapons in 3, Invasion in Reach) but also some bad ones (see above). Halo is at its best when it is leading the industry on some change rather than following it. So that's why rechargeable health, equipment, and uneven starts are bad innovations and dual-wielding, detachable heavy weapons, and Invasion are good innovations. So I understand and agree with a lot of the frustrations with 4 and 5, since 4 followed industry trends of killstreaks and too many unlockables, and 5 followed industry trends on mobility and microtransactions *shudder*.
My problem with 4 and 5's multiplayer is that they weren't really industry leaders or trendsetters in any way, they were pretty much exclusively trend followers. Halo 1, 2, and to a lesser extent 3 and Reach all created something new and interesting that inspired other games to follow, but 3 and Reach were the start of this decline of creativity on the multiplayer side. And that's a natural part of the evolution of a franchise IMO. The same thing happened with Return of the Jedi. It was a great and beloved movie but it wasn't anywhere near the creativity and inventiveness of Star Wars and The Empire Strikes Back. I think if Bungie had left Halo after 2 or 3 games and went to work on Destiny, then that game could have been a true industry leader, but when it came out Bungie had already used a lot of ideas on 3, ODST, and Reach, and the industry had time to independently develop a lot of the ideas Bungie had for Destiny so it wasn't as fresh as it could have been when it launched.
So in conclusion, I think it's absolutely inevitable for a long-running franchise to lose steam due to a lack of creativity. Nintendo is the big big big exception to this, as Breath of the Wild is one of the most exciting games to have come out in recent memory, and Super Mario Odyssey had a lot of great new ideas as well. But not everyone can be Nintendo, and I think when it comes to Halo multiplayer it's not immune to this natural phenomenon.
As for single player (which is more important to me as a Halo fan), Halo hasn't really been in the same trend-following rut as it has with multiplayer. I'm a bit worried that 343i will follow a lot of tired open-world looter shooter trends, but am optimistic that they'll find a way around a lot of the headache inducing ones like Breath of the Wild did. Similarly, multiplayer being free to play does not assuage my fears of trend-following but I'll reserve judgement until we find out more since we know very little about it at this point.