Halo is a business, in the end of things, and it's important to look at the competition. Ignoring competition is asking to fail. Especially when they're large franchises like Call of Duty that take away your market share. Now, what's important is how you overtake that franchise. Copying is obviously not always the best way to go, or the way to go. We know Halo shouldn't do that.

343 learned doing that is dangerous (Halo 4), and them learning showed with Halo 5 thus far. The learning being how they got rid of so much of what Halo 4 added on, and getting back to the aspects a lot of fans enjoyed about the previous Halo games. Yes, there is also still some changes. Changes should be expected to happen as a franchise that doesn't change can become stale and fall downhill overtime (Pokemon before it went 3D/Mega Evolutions/etc.). It's changes that make sense that should happen -- educated, careful changes. Like the small additions CA made with levels like Sanctuary in H2A.

TLDR; It's not the problem to want to be successful like COD. It's the problem to want to be successful like COD by being Call of Duty. That's not always the answer.