I will strongly disagree with a lot of what you are saying, but actually, I just can't figure where you put yourself on the Halo spectrum-gameplay-wise.
Each generation see a new top game that will outmatch other. Halo was the top dog during two generations of console, it's okay for this franchise to not be number one anymore. So at least, keep this franchise as it was so it can please the already existing community and still be unique in the market. People will or will not be interested, but it's better to please you already existing consumer than trying to please everyone. It is the best way to please no one actually.
You can have a very simple formula when it comes to game mechanics and make the level design enhance it and brings new things, be it gametypes or way to improve your skill on already existing maps.
Look at games like Tramaniac, DOOM, Distance (from Refrect Studio) or Rocket League. Those games have very simple mechanics that have been enhanced by the level design or the gametype they were providing while staying true to what they are. They add content on a regular-basis, but not like every months, and they still have a descent amount of players coming back everyday.
I'm not talking about huge success, just good one that keep an healthy community alive while not trying to chase every new trends. New games are here to bring new gameplay mechanics, old game just should improve their own formula (latest Zelda is a very good example of this).
To this day, I can't think of a popular FPS that plays like Halo, and I play a ton of games. Halo is still unique, and that's okay if it's not the number one selling franchise anymore, don't prostitue Halo.
Every game will come to a day it will be boring because you played it during years, Halo isn't different. Overwatch and Battleground will be blurred out in a few years because trends change and people just want to enjoy new things. And we all heard that what is old is new again.
The simple Halo formula will rise again one day, and it will bring new players to this.
As I already say some times ago, I wish 343 would give us a Halo Online on Xbox that would evolved and having new content during like ten years with MT for Warzone or Invasion like gametypes.
People being only engage by content to unlock will leave the same soon enough if your gameplay isn't good, they doesn't care how many armors or skins you can unlock. You have to hook them with gameplay because even thinking adding stuff to unlock. Emotional engagement is way more important than esthetic one. I would even prefer to have more PvE content than PvP.
But still, I'm curious, what would be the top Halo game to you Vincent?