Tl;dr you can change the mechanics of Halo without changing the experience, and that satisfies people like me who are supposedly against change.
The “Halo” that so-called “purists” (like me) want to maintain is qualitative, not quantitative. It doesn’t actually need to conflict with the wishes of the rest of the community. Actually, the skin, bones and muscle of Halo should evolve to further suit and protect the heart, soul and mind of Halo, but Halo’s heart, soul and mind need no such change.
In this poor analogy, Halo’s “heart, soul and mind” means, respectively: the physical feeling of playing Halo (the adrenaline and pace of the game), the emotional feeling (anger at a competitive loss, dopamine rewards after a win or promotion) and the mental feeling of very slowly, gradually improving your understanding of Halo’s various metagames (map positioning, strafing, communication, or just pure mindset).
These are the cornerstones of the franchise for us “purists”. None of this goes against the other side of Halo.
In Halo 2, people complained about the BR being easy to use, dual-wielding cluttering and convoluting the sandbox and 4sk not being the best option for Halo. We’ve all seen the “Halo 2 is noobified” video, for example. Those are valid arguments. What people didn’t complain about was the shift from 2v2 to 4v4 play. They didn’t complain about MK VI. They didn’t even complain about the distinct new style of maps, particularly the shift towards less modular, more symmetrical maps with slightly more open sightlines. Those are changes to the skin, bones and muscle of Halo. They don’t actually matter too much. The subjective experience of Halo gameplay was maintained - for the most part, at least.
The goal of 343 should be to make a venn diagram of what “purists” and what others want, and implement what’s found in the overlapping area. The first go-to features that everyone can agree on are external features like spectator mode, Forge, Theater, mod support, a server browser, clans, seasonal divisional ranks, and so on. These are huge. When it comes to gameplay, there’s a high FoV, higher movement speed, meaningful medals, ranked/unranked division, a sandbox of weapons that all fulfill their own roles and a general crisp, responsive feeling.
Those things alone are more than enough for any sequel.