You bring up great points but you can't have this discussion with out recognizing certain truths.I don't think it's fair to say a game needs to follow a game, I just don't agree. Halo 3 was the most popular online shooter and it didn't get copied that much honestly, but it got beat sure, but halo trying to copy back broke halo imo, no need for it.
Number 1 being COD is not just played by "kids" it makes the conversation dismissive. When brought up in that light it adds nothing to the conversation. COD didn't have to copy Halo it just made the turnstile quicker to get back on the ride.
Halo has its roots in non console gaming. IMO the play style screams mouse and keyboard, this game could have easily sold 5 million opening week if on PC.
Another truth that must be faced is....people moved on from halos play style. Too many other options out there offer a more accessible point of entry. Halo 1, 2 and 3's audience is still out there, I was one of those people that salivated at every release, but like others, I moved on to other choices
You can't get mad at people for that, you can't fault someone for finding something that clicks for them, and you certainly can't fault them for finding halo's steep learning curve as a turn off. Halo will always be a juggernaut , it just may not be one on consoles anymore