Another reason why I think COD will never return to its glory days is down to how the gaming landscape has changed. The golden era of COD around MW2 and BO1 was when deathmatch was still very popular and level progression was a big aspect of people's ongoing enjoyment. Someone going 30-4 was a great gameplay for a YouTube video and there were no competitive stresses. I bet veteran arena shooter players of the millennium days did not like this type of game or its format the same way many don't like COD in its current state.
Nowadays, getting 30 kills is nothing and gameplay commentaries, unless they are giving tips on cheesing or OP weapons, are largely redundant. Someone even getting a double nuke barely gets any attention these days or going 100-0 whilst using knife only. Even the new nuke world record set the other day (72 seconds) didn't bat an eyelid in the community. Sadly it's all about skin culture today and looking good in-game. The e-Sport mentality is being injected into g-fulled players with a rapid meta in terms of mechanics and competitive matchmaking. You can be playing casually yet onscreen it looks like you are sweating it like crazy. Prestige also means little now - a few years ago, players were reaching Prestige 10 level 1000 just after Christmas when even I would struggle to reach just the max prestige over the duration of a year.
Although I can still comfortably hold my own even in the most sweatiest SBMM riddled matches, I think a lot of the fun has been sucked out of the experience because of the way people have been conditioned to play and how the game is designed. It's more mentally exhausting than fun at times.
One other factor which has diminished the fun of the COD titles pretty much the whole of the last gen was disappointing kill/scorestreaks. I can barely name any after Black Ops 2 they were that forgettable. Even ones that returned to the series just didn't have the presence, enjoyement or sheer power like they did in older titles. I really don't know how they got this so wrong and why they haven't bounced back. Could be that it's just another tiring aspect but certainly they sucked from Ghosts onward.