Please explain to me how those things don't have anything to do with the engine. I'd like to read this.
Not sure you know what an engine does, at least in a way that's relevant to what you're saying. Quoting Wiki:
"The core functionality typically provided by a game engine includes a rendering engine (renderer) for 2D or 3D graphics, a physics engine or collision detection (and collision response), sound, scripting, animation, artificial intelligence, networking, streaming, memory management, threading, localization support, and a scene graph."
Sure, glitches in the lobby/matchmaking/menu systems might be a part of it but it's not exactly a vital part and an engine's age doesn't really have anything to do with that. I'm sure the next CoD could use the menu coding from Doom II if it wanted to. Not so much with the graphics rendering or physics aspects.