The bolded statement is true, but only to an extent. While yes, the maps and movement mechanics have drastically changed over time, the guns, killstreaks, and gameplay is so very similar. I mean, no one can sit there and say that the Weevil isn't a ported P90, or the Kn-44 isn't a ported Ak-47. The Hitscan is also exactly the same. For every. single. game. In reality, the Hitscan is what killed many's love for CoD, because literally no changes from BO1-BO3 in Hitscan happened, and bullshit stuff like dying when behind a wall is a thing(When you run behind a wall yet the enemy's bullets still kill you behind the wall). If they made hitscan a bit like CS:GO, where bullets had a bit of realism and weren't 100% accurate, then CoD would probably be a lot more enjoyable for many more. If you've shot a gun irl, you'd know that even if you have the strongest grip, that bullet could go flying in a different direction. CoD doesn't follow this idea, and IMO takes away a lot from it, such as team strategy or thinking smarter. Instead, playing it smart and using the environment to your advantage is seen as cheap and unsportsman-like in CoD, which is sad. Now at the same time, CoD strikes a lot of good places. First and obvious, running around with a sub-machine gun, popping fools like noone's business, is super fun, and just makes me feel like a bad ass. The new class system in BO3 also struck a good note, and things get personal with revenge kills. Cosmetics and a purchase system where it isn't DLC-2-win also struck well. Zombies is very entertaining, and The Giant struck an area I haven't felt in years. The Campain for the BO series is always fun, entertaining, and much better from the clusterfucks that were AW and Ghosts. The Multiplayer is the only problem I see though, because lets face it; If you take 50% of your last product, reskin it, then pass it off for something new, people are going to see through your bullshit.
The problems I have with COD is Activision's laziness. They could've fixed the netcode problem that cause lag from MW2, but it still exists. I love COD 1-4 and WAW, I like the later ones a bit, but they aren't bad, it's just Activision's laziness that keeps the games from its full potential.